Помогите найти баг в алгоритме

 
500 499 
499 498 
123456789 1000 
получится 

321 80
80 481

вместо правильных

792 815
815 422

#include<iostream>
#include<vector>
using namespace std;



typedef vector< vector<int> > Matrix;

Matrix mult(Matrix M1, Matrix M2, const int mod) {
    Matrix M(2,vector<int>(2, 0));
    for (int i = 0; i < 2; ++i) {
        for (int j = 0; j < 2; ++j) {
            for (int k = 0; k < 2; ++k) {
                M[i][j] += M1[i][k]*M2[k][j];
            }
            M[i][j] %= mod;

        }

    }
    /*
    M[0][0] = (M1[0][0]*M2[0][0] + M1[1][0]*M2[0][1])%mod;
    M[0][1] = (M1[0][1]*M2[0][0] + M1[1][1]*M2[0][1])%mod;
    M[1][0] = (M1[0][0]*M2[1][0] + M1[1][0]*M2[1][1])%mod;
    M[1][1] = (M1[0][1]*M2[1][0] + M1[1][1]*M2[1][1])%mod;*/
    /* cout << M[0][0] << ' ' << M[0][1] << endl
        <<  M[1][0] << ' ' << M[1][1] << endl
        <<  "----------" << endl;*/
    return M;
}

Matrix pow(Matrix M, int n, const int mod) {
    if (n > 1) {
        if(n%2 == 1) M = mult(mult(M, M, mod), M, mod);
        else M = mult(M, M, mod);

        return pow(M, n/2, mod);
    }
    else if (n == 0) {
        M[0][0] = 1;
        M[0][1] = 0;
        M[1][0] = 0;
        M[1][1] = 1;
    }
    return M;
}

int main() {
    Matrix M(2,vector<int>(2));
    int n, m;
    while (cin >> M[0][0] >> M[0][1] >> M[1][0] >> M[1][1] >> n >> m) {
        M = pow(M, n, m);
        cout << M[0][0] << ' ' << M[0][1] << endl
            <<  M[1][0] << ' ' << M[1][1] << endl
            <<  "----------" << endl;
    }
}