#include <bits/stdc++.h>

using namespace std;

ifstream fin("cifre.in");
ofstream fout("cifre.out");

#define rnd(a, b) (a + (rand() % (b-a+1)))

int main() {
    srand(time(NULL));

    int a, b, c, k;
    fin >> a >> b >> c >> k;

    int it = 500000, cnt, x, total = 0;
    for(int i=1; i<=it; i++) {
        cnt = 0;
        for(x = rnd(a, b); x; x /= 10)
            cnt += (x % 10 == c);
        total += cnt >= k;
    }

    fout << 1.0 * total / it;
    return 0;
}