/// Preset de infoarena
#include <fstream>
#include <iomanip>

using namespace std;

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

int cif, frecv, v[12], dp[15][15];

int calc(int nr)
{
    int aux = nr, nrc = 0, ans = 0, ramas = frecv;
    bool flag = 0;
    while(aux)
    {
        v[++nrc] = aux % 10;
        aux /= 10;
    }
    if(cif == 0)
    {
        for(int i = nrc - 1; i >= 1; i--)
            ans += 9 * dp[i - 1][frecv];
        flag = 1;
    }
    for(int i = nrc; i >= 1; i--)
    {
        if(v[i] > cif && ramas && !flag)
        {
            ans += dp[i - 1][ramas - 1];
            v[i]--;
        }
        ans += (v[i] - flag) * dp[i - 1][ramas];
        if(v[i] == cif)
            ramas = max(ramas - 1, 0);
        flag = 0;
    }
    if(ramas == 0)
        ans++;
    return ans;

}

int main()
{
    int x, y;
    double ans;
    fin >> x >> y >> cif >> frecv;
    dp[0][0] = 1;
    for(int i = 1; i <= 9; i++)
    {
        dp[i][0] = 9 * dp[i - 1][0];
        for(int j = 1; j <= i; j++)
            dp[i][j] = 9 * dp[i - 1][j] + dp[i - 1][j - 1];
    }
    for(int i = 1; i <= 9; i++)
        for(int j = 8; j >= 0; j--)
            dp[i][j] += dp[i][j + 1];

    ans = 1.0 * (calc(y) - calc(x - 1)) / (y - x + 1);
    fout << fixed << setprecision(4) << ans;
    return 0;
}