#include <fstream>
#include <cstdio>
#include <cstring>
#define i64 long long
 
using namespace std;
 
ifstream cin("cifre.in");
ofstream cout("cifre.out");
 
int A, B, C, K;

double brute(int a,int b,int c,int k) {
	int ret = 0;
	for(int i = a;i <= b;i++) {
		int cc = 0;
		for(int aux = i;aux > 0;) {
			cc += (aux%10 == c);
			aux /= 10;
		}
		if(cc >= k) {
			ret++;
		}
	}
	return 1.0*ret/(b - a + 1);
}
 

int main()
{
    cin>>A>>B>>C>>K;
	cout.precision(4);
	cout<<fixed<<brute(A,B,C,K);
    return 0;
}