#include<stdio.h>
#include<stdlib.h>
#include<time.h>
using namespace std;
int A, B, Cifra, K, Rez, Total, N, Cnt, x;

int main() 
{
	freopen("cifre.in", "r", stdin);
	freopen("cifre.out", "w", stdout);
	
	scanf("%d%d%d%d", &A, &B, &Cifra, &K);
	
//srand( time(0) );
	Total = N = 1005000;

	while( N-- ) 
	{
		x = A + rand() % ( B - A + 1 );

		Cnt = 0;

		while( x ) 
		{
			if( x % 10 == Cifra ) ++Cnt;
			x /= 10;
		}
		if( Cnt >= K ) ++Rez;
	}
	
	printf("%.4lf\n", (double)Rez / (double)Total);
	
	return 0;
}