#include <fstream>
using namespace std;

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

int a, b, c, k, f[10], sol;

int get(int x) {
    for (int i = 0; i < 10; ++i)
        f[i] = 0;
    while (x) {
        f[x % 10]++;
        x /= 10;
    }
    if (f[c] >= k)
        return 1;
    return 0;
}

int main() {
    fin >> a >> b >> c >> k;
    for (int i = a; i <= b; ++i)
        sol += get(i);
    fout << 1.0 * sol / (b-a+1);
}