#include <cstdio>
#include <cstdlib>
#include <ctime>

#define file_in "cifre.in"
#define file_out "cifre.out"

int A,B,C,K,Q,X,ans,nr;

int main(){
	
	freopen(file_in,"r",stdin);
	freopen(file_out,"w",stdout);
	
	scanf("%d %d %d %d", &A, &B, &C, &K);
	
	srand(time(NULL));
	
	Q=2000000;
	
	ans=0;
	while(Q--){
		
		nr=0;
		X=A+rand()%(B-A+1);
		while(X){
			if (X%10==C)
				nr++;
			X/=10;
		}
		
		if (K<=nr)
			ans++;
	}
	double rez=(double)(ans/200); 
	
	printf("%.4lf\n", (rez/10000));
	
	return 0;
	
}