#include <stdio.h>
#include <math.h>

void cut(int&,int,int&);

int main()
{
    int a,b,c,k,var,cont,scan,buff;
    while(1)
    {
    cont = 0;
    scan = 0;
    scanf("%d %d %d %d",&a,&b,&c,&k);
    if(a==0 && b==0 && c==0 && k==0) break;
    
    k = (int)pow(2,k);
    buff = k+1;
    
    for(var=a;var!=b; var+=c,cut(var,k,scan))
    {
	cont++;
	if(var>a && scan==1)
	{	
	    scan = 0;
	    if(buff==var) break; else buff=var;
	}
    }    
    if(buff==var) printf("FOREVER"); else printf("%d\n",cont);    
    }
    return 0;
}

void cut(int &var,int k,int& scan)
{
    if(var>k)
    {
     var=var%k;
     scan = 1;
    }
}