POJ1061青蛙的约会[扩展欧几里得]

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long ll;
ll xx,yy,m,n,l;
void exgcd(ll a,ll b,ll &g,ll &x,ll &y){
    if(b==0){g=a;x=1;y=0;}
    else{exgcd(b,a%b,g,y,x);y-=x*(a/b);}
}
int main(int argc, const char * argv[]) {
    scanf("%lld%lld%lld%lld%lld",&xx,&yy,&m,&n,&l);
    ll x,y,g;
    exgcd(n-m,l,g,x,y);
    if((xx-yy)%g) printf("Impossible");
    else{
        ll c=xx-yy,b=l,a=n-m;
        //printf("%lld",(x*(xx-yy)/g%(l/g)+l/g)%(l/g) );
        c/=g;b/=g;
        printf("%lld",(x%b*c%b+b)%b);
        //printf("%lld",((x%b+b)*c%b+b)%b);
    }
    return 0;
}