# include <cstdio>
using namespace std;
FILE *f=fopen("cifre.in","r"),*g=fopen("cifre.out","w");
char c,k;
bool verifica(int a)
{
	char nr=0;
	do
	{
		if(a%10==c)nr++;
		a/=10;
	}while(a);
	if(nr>=k) return 1;
	else return 0;
}
int main()
{int a,b,i;
float cf;
fscanf(f,"%d %d %c %c",&a,&b,&c,&k);c-='0';k-='0';
for(i=a;i<=b;i++)
{
	if(verifica(i))cf++;
}
fprintf(g,"%.4f",cf/(b-a+1));
}