poj 2115 C Looooops 扩展欧几里德

#include<iostream>
#include<string>
#include<cstring>
#include<algorithm>
#include<cstdio>
using namespace std;
#define ll long long
#define esp 1e-13
const int N=1e4+10,M=1e6+50000,inf=1e9+10,mod=1000000007;
void extend_Euclid(ll a, ll b, ll &x, ll &y)
{
    if(b == 0)
    {
        x = 1;
        y = 0;
        return;
    }
    extend_Euclid(b, a % b, x, y);
    ll tmp = x;
    x = y;
    y = tmp - (a / b) * y;
}
ll gcd(ll a,ll b)
{
    if(b==0)
        return a;
    return gcd(b,a%b);
}
ll pow1(ll x)
{
    ll sum=1;
    for(ll i=0;i<x;i++)
    sum*=2;
    return sum;
}
int main()
{
    ll x,y,i,z,t;
    while(~scanf("%lld%lld%lld%lld",&x,&y,&i,&t))
    {
        if(x==0&&y==0&&i==0&&t==0)
        break;
        ll m=pow1(t);
        ll c=((y-x)%m+m)%m;
        ll j,k;
        if(c%gcd(m,i)==0)
        {
            extend_Euclid(i,m,j,k);
            ll ans=j*(c/gcd(m,i));
            m=m/gcd(m,i);
            printf("%lld\n",(ans%m+m)%m);
        }
        else
        printf("FOREVER\n");
    }
    return 0;
}