#include <fstream>
#include <iostream>
#include <ctime>
#include <cstdlib>
using namespace std;
 
ifstream fin ("cifre.in");
ofstream fout ("cifre.out");
 
const int R = 850000;
 
int a, b, c, k, sol;
 
int main() {
    fin >> a >> b >> c >> k;
    srand ((unsigned) time(0));
    for (int r = 0; r < R; ++r) {
        int x = a + rand() % (b - a + 1), now = 0;
        while (x) {
            if (x % 10 == c)
                now++;
            x /= 10;
        }
        if (now >= k)
            sol++;
    }
    fout << 1.0 * sol / R ;           
}