poj 1061 青蛙的约会

http://poj.org/problem?id=1061

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<map>
#include<queue>
#include<cmath>
#define LL long long

using namespace std;

LL gcd(LL x,LL y)
{
    if(x%y==0)
    return y;
    return gcd(y,x%y);
}
void exgcd(LL a,LL b,LL *x,LL *y)
{
    if(b==0)
    {
        *x=1;
        *y=0;
        return ;
    }
    exgcd(b,a%b,x,y);
    LL t=*x;
    *x=*y;
    *y=(t-(a/b)*(*y));
}
int main()
{
   LL x,y,m,n,L;
   while(cin>>x>>y>>m>>n>>L){
   if(m==n)
   {
       printf("Impossible\n");
       continue;
   }
   LL a=n-m;
   LL b=x-y;
   LL k=gcd(L,a);
   if(b%k!=0)
   {
       printf("Impossible\n");
   }else
   {
       LL x1,y1;
       L/=k;
       a/=k;
       b/=k;
       exgcd(L,a,&x1,&y1);
       y1*=b;
       y1=(y1%L+L)%L;
       cout<<y1<<endl;
   }
   }
   return 0;
}