exgcd

D - C Looooops

http://acm.sdut.edu.cn:8080/vjudge/contest/view.action?cid=212#problem/D

#include <stdio.h>
#include<iostream>
using namespace std;
long long exgcd(long long a,long long  b,long long &x,long long &y)
{
    if(b==0)
    {
        x=1;
        y=0;
        return a;
    }
    long long d;
    long long t;
    d=exgcd(b,a%b,x,y);
    t=x;
    x=y;
    y=t-a/b*y;
    return d;
}
int main()
{
    long long  A,B,C,k;
    while(scanf("%lld %lld %lld %lld",&A,&B,&C,&k))
    {
        if(A == 0 && B==0 && C==0 &&k==0)
            break;
        long long a=C;
        long long b=B-A;
        long long n=(long long)1<<k;
        long long x,y;
        long long d;
        d=exgcd(a,n,x,y);
        if(b%d!=0)
            printf("FOREVER\n");
        else
        {
            x=(x*(b/d))%n;
            x=(x%(n/d)+n/d)%(n/d);
            printf("%lld\n",x);
        }
    }
    return 0;
}