#include <bits/stdc++.h>

using namespace std;

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

const int Lim = 1e5;
const int OpMax = 5e5;

int a, b, c, k;

inline bool Check(int x){
    int digits = 0;
    while(x){
        digits += (x % 10 == c);
        x /= 10;
    }
    return (digits >= k);
}

#define Random(a, b) (a + (rand() % (b - a + 1)))
inline float SolveB(){
    int ways = 0;
    for(int i = 1; i <= OpMax; i++){
        ways += Check(Random(a, b));
    }
    return (1.0 * ways / OpMax);
}

inline float SolveA(){
    int ways = 0;
    for(int i = a; i <= b; i++){
        ways += Check(i);
    }
    return (1.0 * ways / (b - a + 1));
}

int main(){
    srand(time(NULL));
    fin >> a >> b >> c >> k;
    float answer = (b - a <= Lim) ? SolveA() : SolveB();
    fout << answer;
    return 0;
}