poj 2115 C Looooops

参考博客：

https://blog.csdn.net/u012469987/article/details/39041797

https://blog.csdn.net/qq_22902423/article/details/50569835

代码：

#include<iostream>
#include<stdio.h>
using namespace std;
typedef long long ll;
void Exgcd(ll a,ll b,ll &d,ll &x,ll &y){
    if(!b){
        d = a;
        x =1;
        y =0;
    }
    else{
        Exgcd(b,a%b,d,y,x);
        y-=x*(a/b);
    }
    
}
int main(){
    ll a,b,c,k;
    while(~scanf("%lld%lld%lld%lld",&a,&b,&c,&k)&&(a||b||c||k)){
        ll M = 1ll<<k;
        ll d,x,y;
        Exgcd(c,M,d,x,y); 
        ll C = b-a;
        if(C%d!=0){
            cout<<"FOREVER"<<endl;
        }
        else{
            x =(x*(C/d))%M;
            x = (x%(M/d)+(M/d))%(M/d);
            cout<<x<<endl;
        }
    }

    return 0;
}