#include <bits/stdc++.h>
using namespace std;

ifstream in("cifre.in");
ofstream out("cifre.out");

const int NMAX = 1e9;

int K, C;
int comb[20][20], fac[20];

void precalc()
{
    comb[0][0] = 1;
    for( int i = 1;  i <= 18;  ++i ) {
        comb[i][0] = 1;
        for( int j = 1;  j <= 18;  ++j )
            comb[i][j] = comb[i - 1][j] + comb[i - 1][j - 1];

    }
}

int formula(int nr, int fix)
{
    int coef = comb[nr][fix];
    for( int i = 1;  i <= nr - fix;  ++i )
        coef = coef * 9;
    assert(0 <= coef && coef <= 1e9);
    return coef;
}

int formula_2(int nr, int fix)
{
    int ans = 0;
    for( int c = 1;  c <= 9;  ++c ) {
        int liber = nr - 1;
        if( fix - (c == K) < 0 ) continue;
        if( nr - 1 == 0 )
            ans += 1;
        else if( nr - 1 < 0 )
            ans += 0;
        else
            ans += formula(nr - 1, fix - (c == K));
    }
    return ans;
}

int solve(int val)
{
    if( val < 0 )
        return 0;
    if( val == 0 )
        return (C == 0) && (K <= 1);

    int v[12];
    int ind = 0;
    while( val ) {
        v[++ind] = val % 10;
        val /= 10;
    }
    int ans = 0;
    reverse(v+1, v+ind+1);
    for( int cifre = 1;  cifre < ind;  ++cifre ) {
        if( cifre == 1 && C == 0 ) {
            ans += (K <= 1);
        }
        for( int start = 1;  start <= 9;  ++start ) {
            int nrc = (start == K);
            for( int more = 0;  more <= cifre - 1;  ++more ) {
                if( nrc + more >= K )
                    ans += formula(cifre - 1, more);
            }
        }
    }
    for( int pre = 0, nrc = 0;  pre <= ind;   ++pre ) {
        if( pre == ind ) {
            ans += (nrc >= K);
            break;
        }

        for( int c = v[pre + 1] - 1;  c >= 0;  --c ) {
            if( c == 0 && pre == 0 ) continue;

            int cifre = nrc + (c == C);
            int liber = ind - pre - 1;
            for( int more = 0;  more <= liber;  ++more ) {
                if( cifre + more >= K ) {
                    ans += formula(liber, more);
                }
            }
        }

        nrc += (v[pre + 1] == C);
    }
    return ans;
}

int main()
{
    precalc();

    int A, B;
    in >> A >> B >> C >> K;
    out << fixed << setprecision(4) << (solve(B) - solve(A - 1)) * 1.0 / (B - A + 1) << '\n';
    return 0;
}