pku2115 C Looooops

http://poj.org/problem?id=2115

数论，模线性方程

#include <stdio.h>
#include <vector>

using namespace std;

long long extend_gcd(long long a, long long b, long long &x, long long &y)
{
    if(b == 0)
    {
        x = 1;
        y = 0;
        return a;
    }
    long long r = extend_gcd(b, a%b, y, x);
    y -= x*(a/b);
    return r;
}

long long line_mod_equation(long long a, long long b, long long n)
{
    long long x, y;
    long long d = extend_gcd(a, n, x, y);
    if(b%d == 0)
    {
        x %= n;
        x += n;
        x %= n;
        x = x*(b/d)%(n/d);  //最小解
        x = (x%(n/d)+n/d)%(n/d);//最小正整数解
        return x;
    }
    else
    {
        return -1;
    }
}

int main()
{
    long long A, B, C, K;
    long long a, b, n, i, j, r;
    while(scanf("%lld%lld%lld%lld", &A, &B, &C, &K), A||B||C||K) 
    {
        a = C;
        b = B - A;
        n = (long long)1 << K;
        r = line_mod_equation(a, b, n);
        if(r == -1)
        {
            printf("FOREVER\n");
        }
        else
        {
            printf("%lld\n", r);
        }
    }
    return 0;
}