#include <stdio.h>

int main()
{
	unsigned long int a,b,c,z,k,l,K,p;

	while(1)
	{
		scanf("%ld %ld %ld %ld",&a,&c,&b,&k);
		if((a==0)&&(b==0)&&(c==0)&&(k==0)) return 0;
		K=1<<k;
		a=a%K;b=b%K;c=c%K;
		if(a==c) {printf("%d\n",0);continue;}
		if(b==0) {printf("FOREVER\n");continue;}
		l=0; p=1;
		z=(K+c-a)%K;
		while(((b&1)==0)&&((z&1)==0)) {k--;K/=2;z/=2;b/=2;}
		if((b&1)==0) {printf("FOREVER\n"); continue;}
		while(z!=0)
		{
		  if(z&1) {l+=p;z=(K+z-b)%K;z=z/2;p*=2;k--;K/=2;}
		  else{z=z/2;p*=2;k--;K/=2;}
		}
		printf("%ld\n",l);
		
		
	}
	return 0;
}