#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==)

    {

        x=;y=;

        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);

        }

    }

}