#include <stdio.h>
#include <string.h>

int dp[15][15][2][2], dig[15], N;

int solve(int X, int C, int K)
{
    int i, j, k, l, c;

    N = 0;
    do
    {
        dig[++ N] = X % 10;
        X = X / 10;
    }
    while (X);

    int st = 1, dr = N, aux;

    while (st <= dr)
    {
        aux = dig[st];
        dig[st] = dig[dr];
        dig[dr] = aux;
        st ++;
        dr --;
    }

    memset(dp, 0, sizeof(dp));
    dp[0][0][0][0] = 1;

    for (i = 0; i < N; i ++)
        for (j = 0; j <= K; j ++)
            for (k = 0; k <= 1; k ++)
                for (l = 0; l <= 1; l ++)
                    for (c = 0; c <= 9; c ++)
                    {
                        int cnt, add, zero;

                        cnt = j;
                        if (c == C)
                        {
                            cnt ++;
                            if (c == 0 && l == 0)
                                cnt --;
                        }
                        if (cnt > K)
                            cnt = K;

                        if (k == 0 && c > dig[i + 1])
                            continue;
                        add = k;
                        if (c < dig[i + 1])
                            add = 1;

                        zero = l;
                        if (c)
                            zero = 1;

                        dp[i + 1][cnt][add][zero] += dp[i][j][k][l];
                    }

    return dp[N][K][0][1] + dp[N][K][1][1];
}

int main()
{
    int A, B, C, K;

    freopen("cifre.in", "r", stdin);
    freopen("cifre.out", "w", stdout);

    scanf("%d%d%d%d", &A, &B, &C, &K);

    double prob = solve(B, C, K) - solve(A - 1, C, K);
    prob = (double)prob / (B - A + 1);

    printf("%lf", prob);
    return 0;
}