#include <stdio.h>

unsigned int MaxVal( int k ) {
    unsigned int maxval = 0;
    for (int i = 0; i < k; i++) {
	maxval = maxval << 1;
	maxval += 1;
    }
    return maxval;
}

int main(void) {
    unsigned int a, b, c, k = 1;
    unsigned int count, maxval;
    unsigned int i;    
    scanf ("%u %u %u %u", &a, &b, &c, &k);    
    
    while (!(a == 0 && b == 0 && c == 0 && k == 0)) { 
	count = 0;
	maxval = MaxVal(k);
		
	i = a;
	
	if (a == b) {
		printf("0\n");
		scanf ("%u %u %u %u", &a, &b, &c, &k);
		continue;
	}
	
	if (c == 0) {
	    printf("FOREVER\n");
	    scanf ("%u %u %u %u", &a, &b, &c, &k);
	    continue;
	}
	    
	    
	
	while((i & maxval) != b) {
	    i += c;
	    //printf("--%u %u %u %u\n", a, b, c, i);
	    //i &= maxval;
	    if ((i & maxval) == a)
		break;
	}
	
	count = (i - a) / c;
	i &= maxval;
	
	if (i == a) {
	    if (i == b) {
		printf("0\n");
	    } else {
		printf("FOREVER\n");
	    }
	} else {
	    printf("%u\n", count);
	}
	scanf ("%u %u %u %u", &a, &b, &c, &k);
    }
    return 0;

}