#include <bits/stdc++.h>
 
using namespace std;
 
 
 
int a, b, c, k;
 
inline int lgput(int x, int p){
 
    int ans = 1, aux = x;
 
    for(int i = 0; (1 << i) <= p ; ++i){
 
        if((1 << i) & p) ans *= aux;
 
        aux *= aux;
 
    }
 
    return ans;
 
}
 
inline int comb(int n, int k){
 
    int ans = 1;
 
    for(int i = 2; i <= n ; ++i)
 
        ans *= n;
 
    for(int i = 2; i <= k ; ++i)
 
        ans /= i;
 
    for(int i = 2; i <= n - k ; ++i)
 
        ans /= i;
 
    return ans;
 
}
 
inline int solve(int x, int c, int k){
 
    int nrc = 0, aux = x, a[11];
 
    while(aux > 0) a[++nrc] = aux % 10, aux /= 10;
 
 
 
    if(x == 0 && c == 0 && k == 1) return 1;
 
    if(k > nrc) return 0;
 
 
 
    int d[15][15];
 
    memset(d, 0, sizeof(d));
 
    d[1][0] = 8;
 
    if(c == 0) ++d[1][0], d[1][1] = 0;
 
    else d[1][1] = 1;
 
    for(int i = 2; i < nrc ; ++i){
 
        d[i][0] = d[i - 1][0] * 9;
 
        for(int j = 1; j <= i ; ++j)
 
            d[i][j] = d[i - 1][j] * 9 + d[i - 1][j - 1];
 
    }
 
    int ans = 0;
 
    for(int i = 1; i < nrc ; ++i){
 
        for(int j = k; j <= i ; ++j)
 
            ans += d[i][j];
 
    }
 
    memset(d, 0, sizeof(d));
 
 
 
    d[1][0] = 9; d[1][1] = 1;
 
    for(int i = 2; i < nrc ; ++i){
 
        d[i][0] = d[i - 1][0] * 9;
 
        for(int j = 1; j <= i ; ++j)
 
            d[i][j] = d[i - 1][j] * 9 + d[i - 1][j - 1];
 
    }
 
    for(int i = 1; i <= nrc / 2 ; ++i)
 
        swap(a[i], a[nrc - i + 1]);
 
 
 
    for(int i = 1; i <= nrc ; ++i){
 
        int cr = nrc - i;
 
        if(k > cr + 1) break ;
 
        if(k == 0){
 
            if(i == 1) ans = ans + (a[i] - 1) * lgput(10, cr);
 
            else ans = ans + a[i] * lgput(10, cr);
 
            if(cr == 0) ++ans;
 
            continue ;
 
        }
 
        if(cr == 0){
 
            if(k == 0) ans = ans + a[i];
 
            else if(k == 1) ans = ans + (c <= a[i]);
 
            continue ;
 
        }
 
        if(i == 1){
 
            if(c == 0){
 
                int k2 = k;
 
                for(int j = k2; j <= cr ; ++j)
 
                    ans = ans + d[cr][j] * (a[i] - 1);
 
            }
 
            else{
 
                int k2 = k;
 
                if(c < a[i]){
 
                    ///il pun pe c
 
                    --k2;
 
                    for(int j = k2 + (k2 < 0); j <= cr ; ++j)
 
                        ans = ans + d[cr][j];
 
                    ++k2;
 
                }
 
                ///nu il pun pe c
 
                int nr = a[i] - 1 - (c < a[i]);
 
                if(nr < 0) nr = 1;
 
                for(int j = k2; j <= cr ; ++j)
 
                    ans = ans + d[cr][j] * nr;
 
            }
 
        }
 
        else{
 
            int k2 = k;
 
            if(c < a[i]){
 
                ///il pun pe c
 
                --k2;
 
                for(int j = k2 + (k2 < 0); j <= cr ; ++j)
 
                    ans = ans + d[cr][j];
 
                ++k2;
 
            }
 
            ///nu il pun pe c
 
            int nr = a[i] - (c < a[i]);
 
            if(nr < 0) nr = 0;
 
            for(int j = k2; j <= cr ; ++j)
 
                ans = ans + d[cr][j] * nr;
 
        }
 
 
 
        if(a[i] == c && k > 0) --k;
 
    }
 
    return ans;
 
}
 
int main()
 
{
 
    freopen("cifre.in", "r", stdin);
 
    freopen("cifre.out", "w", stdout);
 
    scanf("%d%d%d%d", &a, &b, &c, &k);
 
    int nr = solve(b, c, k) - solve(a - 1, c, k);
 
    double sol = (double)nr / (double)(b - a + 1);
 
    printf("%f", sol);
 
    return 0;
 
}