POJ1061 青蛙的约会

/*
 POJ1061 青蛙的约会
 http://poj.org/problem?id=1061
 扩展欧几里得

 原题为求t使得,存在l满足
    (x+mt)-(y+nt)=cl
    即求
    cl+(n-m)t=x-y
 *
 *
 *
 */

#include <cstdio>
#include <cmath>
using namespace std;
long long ex_gcd(long long a,long long b,long long &x,long long &y)//solve x,y in a*x+b*y=ex_gcd(a,b,x,y)=gcd(a,b);
{
    if(b==0LL)
    {
        x=1LL;
        y=0LL;
        return a;
    }
    long long ans=ex_gcd(b,a%b,x,y);
    long long tmp=x;
    x=y;
    y=tmp-a/b*y;
    return ans;
    //x = x0 + (b/gcd)*t
    //y = y0 – (a/gcd)*t
     
}
int main()
{
    long long x,y,m,n,l;
    scanf("%lld%lld%lld%lld%lld",&x,&y,&m,&n,&l);
    long long a=l,b=n-m;
    l=x-y;
    long long xx,yy;
    long long gcd=ex_gcd(a,b,xx,yy);
    if(l%gcd)
    {
        printf("Impossible
");
        return 0;
    }

    long long r=l/gcd;
    //while(xx<=0)
    //{
        //xx=xx+b/gcd;
        //yy=yy-a/gcd;
    //}
    yy=yy*r;
    a=a/gcd;
    long long t=yy%a;
    while(t<0)
        t+=a;
    printf("%lld
",t);
    return 0;
}