#include <bits/stdc++.h>

using namespace std;

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

int A, B, C, K, dp[11][11][2];

int f(int x) {
    if(x < 0)
        return 0;
    for(int i = 0; i < 11; ++i)
        for(int j = 0; j < 11; ++j)
            for(int k = 0; k < 2; ++k)
                dp[i][j][k] = 0;
    vector<int> a;
    while(x) {
        a.emplace_back(x % 10);
        x /= 10;
    }
    reverse(a.begin(), a.end());
    int N = a.size();
    dp[0][0][0] = 1;
    for(int i = 0; i < N; ++i)
        for(int j = 0; j < N; ++j)
            for(int lower = 0; lower < 2; ++lower)
                for(int c = 0; c < 10; ++c) {
                    if(!lower && c > a[i])
                        continue;
                    int new_j = j;
                    bool new_lower = lower | (c < a[i]);
                    if(c == C)
                        ++new_j;
                    dp[i + 1][new_j][new_lower] += dp[i][j][lower];
                }
    int ans = 0;
    for(int i = K; i <= N; ++i)
        ans += dp[N][i][0] + dp[N][i][1];
    return ans;
}

int main() {
    fin >> A >> B >> C >> K;
    fout << fixed << setprecision(4) << (double)(f(B) - f(A - 1)) / (B - A + 1) << '\n';
}