#include <stdio.h>

unsigned int A,B,C,k,x,a1,a2;
long long base, tY, y;
int check,i, temp;

int main() {

    while (1) {
    
	scanf("%d %d %d %d\n",&A,&B,&C,&k);
	if (A==0 && B==0 && C==0 && k==0) break;
	base = 1<< k;
	y = (B % base) - (A % base);
	while (y<0) y = y + base;
	while (y>=base) y = y - base;
	C = C % base;
	if ((C==0) && (y==0)) { printf("0\n"); continue; }
	while (((C % 2) == 0) && ((y % 2) == 0)) 
	    { C = C/2; y = y/2; k--; }
	    
	/* printf("--- %d %d %d\n",C,y,temp); */
	
	a1=0; base=2;
	
	for(i=0;i<k;i++) {
	    check=0;
	    a2 = a1 + (base/2);
	    tY = y % base;
	    if (((a1*C)%base) == tY) { check=1; }
	    if ((check==0) && (((a2*C)%base) == tY)) { a1=a2; check=1; }
	    if (check==0) { check=-1; break; }
	    /* printf("* %d -> %d\n",a1,y); */
	    base*=2;
	}
	if (check>=0) { printf("%d\n",a1); }
	else { printf("FOREVER\n"); }
	
	
    }



    return 0;
}