/*
    "care a facut teste cu Lattice reduction attack e ciudat"
    "linistiti va putin"
    - 2023 -
*/
#include<bits/stdc++.h>
#define debug(x) cerr << #x << " " << x << "\n"
#define debugs(x) cerr << #x << " " << x << " "
#pragma GCC optimize("Ofast")
#define int long long

using namespace std;

int dp[12][12][2];
int vlim[12];
int c,k;

int calc(int x)
{
    int cnt,len,newcif,app,ans;
    cnt=0;
    do
    {
        cnt++;
        vlim[cnt]=x%10;
        x=x/10;
    }
    while (x);
    reverse(vlim+1,vlim+1+cnt);
    for (len=1; len<=cnt; len++)
    {
        for (app=0;app<=cnt;app++)
        {
             dp[len][app][0]=0;
             dp[len][app][1]=0;
        }
    }
    for (len=1; len<=cnt; len++)
    {
        if (len!=1)
        {
            for (app=0;app<=len;app++)
            {
                 if (vlim[len]==c && app!=0)
                 {
                     dp[len][app][1]+=dp[len-1][app-1][1];
                 }
                 else
                 {
                     dp[len][app][1]+=dp[len-1][app][1];
                 }
            }
            for (newcif=0; newcif<=9; newcif++)
            {
                for (app=0; app<=len; app++)
                {
                    if (newcif<vlim[len])
                    {
                        if (newcif==c && app!=0)
                        {
                            dp[len][app][0]+=dp[len-1][app-1][0];
                            dp[len][app][0]+=dp[len-1][app-1][1];
                        }
                        else
                        {
                            dp[len][app][0]+=dp[len-1][app][0];
                            dp[len][app][0]+=dp[len-1][app][1];
                        }
                    }
                    else
                    {
                        if (newcif==c && app!=0)
                            dp[len][app][0]+=dp[len-1][app-1][0];
                        else
                            dp[len][app][0]+=dp[len-1][app][0];
                    }
                }
            }
        }
        else
        {
            for(newcif=0; newcif<=9; newcif++)
            {
                if (newcif==vlim[len] && newcif==c)
                    dp[len][1][1]=1;
                else if (newcif==c && newcif<vlim[len])
                    dp[len][1][0]=1;
                else if (newcif<vlim[len])
                    dp[len][0][0]++;
                else if (newcif==vlim[len])
                    dp[len][0][1]=1;
            }
        }
    }
    ans=0;
    for(app=k;app<=cnt;app++)
    {
        ans+=dp[cnt][app][0];
        ans+=dp[cnt][app][1];
    }
    return ans;
}

signed main()
{
    ifstream fin("cifre.in");
    ofstream fout("cifre.out");
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    long long a,b,favorabil,total;
    long double probabilitate;
    fin >> a >> b >> c >> k;
    if (a!=0)
    {
        favorabil=calc(b)-calc(a-1);
        total=b-a+1;
    }
    else
    {
        favorabil=calc(b);
        total=b;
    }
    probabilitate=1.0*favorabil/total;
    fout << probabilitate;
}