Codeforces 787 A The Monster 扩欧

#include <iostream>
#include <cstdio>
#include <map>
#include <iterator>
#include <string>
#include <algorithm>
#include <vector>
#include <cmath>
#include <cstring>
using namespace std;
typedef long long ll;

void exgcd( int a, int b, int &g, int &x, int &y ) {
    if( !b ) { x = 1, y = 0, g = a; }
    else {
        exgcd(b, a%b, g, y, x);
        y -= a / b * x;
    }
}

int main() {
    int a, c1, b, c2;
    cin >> a >> c1 >> b >> c2;
    int c = c2 - c1;
    int g, x, y;
    exgcd(a, b, g, x, y);
    if( c % g ) {
        cout << -1 << endl;
    }
    else {
        int b1 = b / g;
        x *= (c/g);
        x = (x%b1+b1)%b1;
        while( (c-a*x)/b > 0 ) x += b1;
        cout << a * x + c1 << endl;
    }
    return 0;
}