POJ1061青蛙的约会(扩展欧几里德算法)

#include<cstdio>
#include<cstring>
using namespace std;

long long extend_gcd(long long a, long long b, long long &x, long long &y)
{
    if(a == 0 && b == 0) return -1;
    if(b == 0){x = 1; y = 0; return a;}
    long long d = extend_gcd(b, a%b, y, x);
    y -= a/b*x;
    return d;
}

int main()
{
    long long x,y,m,n,l;
    while(~scanf("%lld%lld%lld%lld%lld", &x, &y, &m, &n, &l))
    {
        long long b1,c;
        if(n-m > 0)
            {
                b1 = n-m;
                c = x - y;
            }
        else
            {
            b1 = m-n;
            c = y-x;
            }
        long long x1,y1;
        long long dd = extend_gcd(l, b1, x1, y1);
        if(c%dd)
            printf("Impossible\n");
        else
            {
                long long yy = c/dd*y1;

                long long aa = l/dd;

                yy = yy%aa;

                int cnt = 0;
                while(yy < 0)
                {
                    cnt--;
                    yy = yy - cnt*aa;
                }
                printf("%lld\n",yy);
            }

    }

    return 0;
}