poj 2115 C Looooops

#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

typedef long long LL;

void Ex_Gcd(LL a, LL b, LL &d, LL &x, LL &y)
{
    if(b == 0)
    {
        x = 1;
        y = 0;
        d = a;
        return ;
    }
    else
    {
        Ex_Gcd(b, a%b, d, x, y);
        LL temp = x;
        x = y;
        y = temp - (a/b)*y;
    }
}


int main()
{
    LL a, b, c, k;
    LL d, x0, y0;
    while(scanf("%I64d%I64d%I64d%I64d", &a, &b, &c, &k))
    {
        if((a+b+c+k) == 0)
            break;
        LL temp = c;
        c = b-a;
        a = temp;
        b = (LL)1<<k;//这一块要注意 LL 不能省略
        Ex_Gcd(a, b, d, x0, y0);
        if(c % d != 0)
        {
            printf("FOREVER\n");
            continue;
        }
        else
        {
            LL t = b/d;
            x0 = x0 * (c/d);
            x0 = (x0%t + t)%t;
            printf("%I64d\n", x0);
        }
    }
    return 0;
}