#include<iostream>

#include<cstdio>

#include<cstring>

#define ll long long

using namespace std;

ll a,b,x,y,r,A,B,C,k,g;

ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}

void exgcd(ll a,ll b,ll &x,ll &y)

{

  if(!b){x=;y=;return;}

  exgcd(b,a%b,y,x);y-=a/b*x;

}

int main()

{

  while()

    {

      scanf("%lld%lld%lld%lld",&A,&B,&C,&k);

      if(!A&&!B&&!C&&!k)return ;

      a=C;b=(1ll<<k);r=B-A;

      ll g=gcd(a,b);

      if(r%g){printf("FOREVER\n");continue;}

      a/=g;b/=g;r/=g;/////////

      exgcd(a,b,x,y);

      printf("%lld\n",(x%b*r%b+b)%b);//

    }

}