matrix multiplication

//matrix multiplication
struct matrix {
    ll mat[6][6];
    matrix(){
        for(int i = 0;i<6;i++){
            for(int j = 0;j<6;j++)
                mat[i][j] = 0;
        }
    }
};

matrix mul(matrix &a,matrix &b){
    matrix res;
    for(int i = 0;i<6;i++){
        for(int j = 0;j<6;j++){
            for(int k = 0;k<6;k++){
                res.mat[i][j] += (a.mat[i][k] * b.mat[k][j])%M;
                res.mat[i][j] %= M;
            }
        }
    }
    return res;
}

matrix expo(matrix &base,ll power){
    if(power==1){
        return base;
    }
    if(power&1ll){
        matrix temp = expo(base,power-1);
        return mul(base,temp);
    }
    matrix temp = expo(base,power/2);
    return mul(temp,temp);
}