poj2115 C Looooops——扩展欧几里得

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
typedef long long ll;
ll A,B,C,k,a,b,x,y,g,s;
ll gcd(ll a,ll b){return a%b?gcd(b,a%b):b;}
void exgcd(ll a,ll b,ll &x,ll &y)
{
    if(!b){x=1; y=0; return;}
    exgcd(b,a%b,x,y);//最近总是忘写这个啊...无力... 
    ll t=x; x=y; y=t-a/b*y;
}
int main()
{
    while(1)
    {
        scanf("%lld%lld%lld%lld",&A,&B,&C,&k);
        if(!A&&!B&&!C&&!k)return 0;
        a=C; b=1ll<<k; g=gcd(a,b); s=(B-A+b)%b;
        if(s%g){printf("FOREVER\n"); continue;}
        a/=g;//()
        b/=g;//!!
        s/=g;//!
        exgcd(a,b,x,y);//ax+by=g
        x=(x*s%b+b)%b;
        printf("%lld\n",x);
    }
}