#include <stdio.h>

int main ()
{
	freopen("cifre.in", "r", stdin);
	freopen("cifre.out", "w", stdout);
	int a, b, c, k, i, x, n, s=0;
	scanf("%d %d %d %d", &a, &b, &c, &k);
	for (i=a; i<=b; i++) {
		x=i;
		n=0;
		while (x>0) {
			if (x%10==c) {
				n++;
			}
			x/=10;
			if (n>=k) {
				s++;
				continue;
			}
		}
	}
	float p = (float) s / (float) (b-a+1);
	printf("%.4lf", p);
	return 0;
}