#include<stdio.h>

long long extgcd(long long a,long long b,long long &x,long long &y)
{
    if(b==0)
    {
        x=1;
        y=0;        
        return a;    
    }        
    long long d=extgcd(b,a%b,x,y);
    long long t=x;
    x=y;    
    y=t-a/b*y;    
    return d;    
}
int main()
{
    long long a,b,c,k,n,p,x,y,d,r,ans;
    while(~scanf("%lld%lld%lld%lld",&a,&b,&c,&k))   
    {
        if(a==0&&b==0&&c==0&&d==0)
            break;
        n=b-a;
        p=1;
        p=1ll<<k;
        d=extgcd(c,p,x,y);
        if(n%d)
            printf("FOREVER\n");
        else
        {
            r=n/d;
            ans=r*x;
            if(ans<0)
                ans=(ans%(p/d)+(p/d))%(p/d);    
            printf("%lld\n",ans);
        } 
    }
    return 0;
}