POJ 1061 —— 青蛙的约会

#include <cstdio>
#include <iostream>
#include <algorithm>
using namespace std;

typedef long long LL;

int exgcd(int a, int b, LL &x, LL &y)
{
    if(b==0) {
        x = 1;
        y = 0;
        return a;
    }    
    int d = exgcd(b, a%b, x, y);
    int t = x;
    x = y;
    y = t-a/b*y;
    return d;
}

bool f(int a, int b, int c, LL &x, LL &y)
{
    int d = exgcd(a, b, x, y);
    
    if(c%d)    return 0;
    int k = c/d;
    x *= k, y *= k;
    
    int t = b/d;
    while(x<0) {
        x += t;
    }
    while(x-t >=0) {
        x -= t;
    }
    return 1;
}

int main ()
{
    LL x, y;
    int m, n, L;
    scanf("%d%d%d%d%d", &x, &y, &m, &n, &L);
    if(n<m) {
        swap(n, m);
        swap(x, y);
    }
    
    if(f(n-m, L, x-y, x, y)) {
        printf("%lld\n", x);
    } else {
        printf("Impossible\n");
    }
    return 0;
}