题目链接：http://codeforces.com/contest/787/problem/A
题意：给你a,b,c,d并且有两个数列，一个数列是b, b+a, b+2*a, ”’ ，另外一个是 d , d + c, d + 2*c,”’，现在问你这两个数列最小的相等的值是什么？
解法：
You need to find out if there are non-negative integers like i and j such ai + b = cj + d and i or j (or both) is minimized. It’s easy to show that if a, b, c, d ≤ N, and such i and j exist, then i, j ≤ N, so you can iterate over i and check if such j exists.

我直接两重for循环暴力的。
复杂度O(n^2)

//CF 787A

#include <bits/stdc++.h>
using namespace std;

int main()
{
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int ans = 0x3f3f3f3f;
    for(int i = 0; i <= 1000; i++){
        for(int j = 0; j <= 1000; j++){
            if(b+a*i == d+c*j){
                ans = min(ans, b+a*i);
            }
        }
    }
    if(ans == 0x3f3f3f3f) puts("-1");
    else cout << ans << endl;
}