///Jmekeria suprema!
#include <bits/stdc++.h>
using namespace std;

const int PRECISION = 1e6;

inline int cdig(int num, int dig) {
    int ans = 0;

    do {
        if(num%10==dig)
            ++ans;
        num/=10;
    } while(num);

    return ans;
}

int main(void) {
    freopen("cifre.in", "r", stdin);
    freopen("cifre.out", "w", stdout);
    srand(time(NULL));

    int a, b, c, k, t, e = 0;

    scanf("%d%d%d%d",&a,&b,&c,&k);

    for(int i=0; i<PRECISION; ++i) {
        t = a + rand() % (b - a + 1);
        if(cdig(t, c)>=k)
            ++e;
    }

    printf("%.4f\n",double(e)/PRECISION);

    fclose(stdin);
    fclose(stdout);
}