#include <stdio.h>

long long a,b,c,x,moc,exp;
long long e;

int main()
{
	int i;
	
	while (1)
	{
		scanf("%lld %lld %lld %lld", &a, &b, &c, &e);
		if (e==0) break;
		exp=((long long)1)<< e;
		b=(b-a+exp)%exp;
		
		while (b%2==0 && c%2==0) 
		{
			b = b >> 1;
			c = c >> 1;
			e--;
		}
		if (c%2==0 && b%2==1) 
		{
			printf("FOREVER\n");
			continue;
		}

		x=0; moc=1; exp=((long long) 1) << e;
		
		for ( i=0; i<e; i++ ) 
		{
			if (b%2==1)
			{
				x+=moc;
				b=(b-c+exp)%exp;
			}
			b = b >> 1; moc=moc << 1;
		}

		printf("%lld\n", x);
	}	

	return 0;
}