#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;

}