poj2115-C Looooops -线性同余方程

#include <cstdio>
#include <cstring>

#define LL long long

using namespace std;
//A+n*C = B mod 2^k
//n*C = B-A mod 2^k


LL A,B,C,MOD;
int k;

LL ExGCD(LL a,LL b,LL &x,LL &y)
{
    LL d,t;
    if(b==0)
    {
        x=1;y=0;
        return a;
    }
    d = ExGCD(b,a%b,x,y);
    t=x;x=y;y=t-a/b*y;
    return d;
}

int main()
{
    while(scanf("%I64d%I64d%I64d%d",&A,&B,&C,&k) && (A||B||C||k))
    {
        LL x,y;
        LL a=C,b=B-A;
        MOD = 1LL<<k;
        LL d = ExGCD(a,MOD,x,y);
        if(b%d )
        {
            printf("FOREVER\n");
        }
        else
        {
            x=(x*(b/d))%MOD;
            x=(x%(MOD/d)+MOD/d)%(MOD/d);
            printf("%I64d\n",x);
        }
    }
}