#include<fstream>
#include<iomanip>
#include<cstring>
using namespace std;
int a, b, c, k, i, j, nr, x, nrc;
int d[10][10], f[10], s[10][10], v[10], w[10], s1[10][10], d1[10][10];
double sol;
ifstream fin("cifre.in");
ofstream fout("cifre.out");
int fr(int x){
    int sol = 0, nr = 0;
    int aux = x;
    while(aux != 0){
        v[++nr] = aux % 10;
        aux /= 10;
    }
    sol += s[nr - 1][k];
    memset(w, 0, sizeof(w));
    w[nr] = 1;
    int j = nr;
    while(j > 1){
        while(w[j] == v[j] && j > 0){
            j--;
        }
        if(j <= 1){
            break;
        }
        int nrc = 0;
        for(int i = j; i <= nr; i++){
            if(w[i] == c){
                nrc++;
            }
        }
        if(c == 0){
            sol += s1[j - 1][max(0, k - nrc)];
        }
        sol += s[j - 1][max(0, k - nrc)];
        w[j]++;
    }
    while(w[1] <= v[1]){
        int nrc = 0;
        for(int i = 1; i <= nr; i++){
            if(w[i] == c){
                nrc++;
            }
        }
        if(nrc >= k){
            sol++;
        }
        w[1]++;
    }
    return sol;
}
int main(){
    fin>> a >> b >> c >> k;
    for(i = 1; i <= 9; i++){
        memset(f, 0, sizeof(f));
        if(c != 0){
            while(f[0] == 0){
                nrc = 0;
                nr = 1;
                for(j = 1; j <= i; j++){
                    if(f[j] == 1){
                        nrc++;
                    }
                }
                for(j = 2; j <= i; j++){
                    if(f[j] == 0){
                        nr *= 9;
                    }
                }
                if(f[1] == 0){
                    nr *= 9;
                }
                d[i][nrc] += nr;
                j = i;
                while(f[j] == 1){
                    f[j] = 0;
                    j--;
                }
                f[j] = 1;
            }
        }
        else{
             while(f[1] == 0){
                nrc = 0;
                nr = 1;
                for(j = 1; j <= i; j++){
                    if(f[j] == 1){
                        nrc++;
                    }
                }
                for(j = 2; j <= i; j++){
                    if(f[j] == 0){
                        nr *= 9;
                    }
                }
                d1[i][nrc + 1] += nr;
                if(f[1] == 0){
                    nr *= 9;
                }
                d[i][nrc] += nr;
                d1[i][nrc] += nr;
                j = i;
                while(f[j] == 1){
                    f[j] = 0;
                    j--;
                }
                f[j] = 1;
            }
        }
    }
    for(i = 1; i <= 9; i++){
        for(j = i; j >= 0; j--){
            s[i][j] = d[i][j] + s[i][j + 1];
            s1[i][j] = d1[i][j] + s1[i][j + 1];
        }
    }
    x = fr(b) - fr(a - 1);
    sol = x * 1.0 / (b - a + 1);
    fout<< setprecision(4) << fixed << sol <<"\n";
    return 0;
}