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

int main()
{
	int a,b,c,d;
	unsigned long l = 0;
	unsigned long count = 0;
	unsigned long n = 0;
	

	
	while(1)
	{
	
	count = 0;
	n = 0;
	

	scanf("%d %d %d %d",&a,&b,&c,&d);
	if(a == 0 && b == 0 && c == 0 && d == 0) break;

	
	
	l = pow(2,d);
	
	
	for (n = a; n != b; n += c)
	{
	   if (n >= l) n = n-l;
	   count++;
	
	   if (count > l-1) break;		
	}
	
	
	if (count > l-1)
		printf("FOREVER\n");
	else
		printf("%ld\n",count);

	}
	return 0;
}