Thread 1: EXC_BAD_ACCESS (code=2, address=0x16f603ff8)

I'm currently writing a code to achieve the RSA encryption via C++, and I stumbled across this error message when I was running it.

The first part of my code is to generate random prime numbers based on the digit from the input for the encryption. For example, if you enter 6 then the code will generate two prime numbers, M and m, between 100000 to 999999. The "fin" variable equals (M-1)*(m-1), and based on "fin" I will create an int array the size of [fin+1] in the "Random e" section. However, if I enter a number greater than 3 ( which means "fin" will go beyond 6 digits) the crash will occur and the error message will show.

I can't seem to find the problem anywhere, and if my input is no greater than 3 then the code will work exactly as I wanted it to. Is there something I missed? Thank you very much.

Xcode [14.3.1]

#include <iostream>
#include <stdlib.h>
#include <time.h>

using namespace std;
    

    long long int gcd(long long int x, long long int y)
    {
        
        for (long long int t; y!=0; x=t)
        {
            t=y;
            y=x%y;
        }
        return x;
    }

    long long int mod(long long int x, long long int y, long long int n)
    {
        if (x%n==0)
        {
            return 0;
        }
        
        long long int result;
        for (result=1; y>1; x=(x*x)%n)
        {
            if (y%2==1)
            {
                result = (result * x)%n;
                y=(y-1)/2;
            }
            else if (y%2==0)
            {
                y=y/2;
            }
        }
        x=(x*result)%n;
        return x;
    }

    int main()
    {
        // Random Prime Numbers------------------------------//
       
        cout << "Input digits" << endl;
        int k;
        cin >> k;
        
        cout << "Input letter for encryption" << endl;
        char f;
        cin >> f;
        
        long long int M=pow(10, k)-1;
        long long int m=pow(10, k-1);
        bool prime[M+1];
        memset(prime, true, sizeof(prime));
    
        
        for(long long int p=2; p*p<=M; p=p+1 )
        {
            if (prime[p] == true)
            {
               for (long long int i=p*p; i<=M; i=i+p)
                    prime[i]=false;
            }
        }
        
        
        int prime2[M+1];
        for (long long int t=0; t<=M; t=t+1)
        {
            prime2[t]=1;
        }
        

        for (long long int p=m, t=1; p<=M; p=p+1)
        {
            if(prime[p]==true)
            {
                prime2[t]=p;
                t=t+1;
            }
        }
        

        long long int h = count(prime, prime+M+1, true);
        long long int l = count(prime, prime+m, true);
        h=h-l;
        
      
        srand(time(NULL));
        M=rand()%h+1;
        m=rand()%h+1;
        if (M==m)
        {
            m=(M*1000)%h+1;
        }
        
        
        if (M>h)
        {
            cout << "a=" << M << " error!" << endl;
        }
        else if (m>h)
        {
            cout << "b=" << m << " error!" << endl;
        }
        else
        {
            M=prime2[M];
            m=prime2[m];
        }
        
        
        
        
        
        // Generating RSA keys----------------------------//
        
        
        long long int n = M*m;
        long long int fin = (M-1)*(m-1);
       

      
            // Generating random e for the public key {e, n}--------//

            long long int number[fin+1];
            for (long long int i=0; i<=fin; i=i+1)
            {
                number[i]=0;
            }
            

            for (long long int i=2, k=1; i<=fin; i=i+1)
            {
                if (gcd(i, fin)==1)
                {
                    number[k]=i;
                    k=k+1;
                }
            }
            

            long long int z = fin+1-count(number, number+fin+1, 0);
        
            srand(time(NULL));
            
            long long int r = rand()%z +1;
            long long int e = number[r];
            
            
    
        long long int e2 = e;
        
        long long int d=0;
        for (long long int x=1, y=0, k=1, t=1; e2%fin!=0; k=1, d=y)
        {
            t=y;
            y=x-y*(e2/fin);
            x=t;
            
            k=fin;
            fin=e2%fin;
            e2=k;
        }

        cout << endl << "The public key is {" << e << " , " << n << "}" << endl;
        cout << "The private key is {" << d << " , " << n << "}" << endl;
        
        // e、d、n  acquired
        
        // Encryption begins----------------------------//

        long long int f2 = (int)f;
        f2 = mod(f2, e, n);
        cout << "The cyphertext is " << f2 << endl;
        
        f2 = mod(f2, d, n);
        f = (char)f2;
        cout << f << endl;
        
        
        
        
    }