#include <stdio.h>
#include <string.h>

long a,b,i,nr=0,r,x;
int v[9],c,k;

int main()
{
freopen("cifre.in","rt",stdin);
scanf("%ld %ld %d %d", &a,&b,&c,&k);
for (i=a;i<=b;++i)
     {
     x=i;
      memset(v,0,sizeof(v));
      while (x)
	    {
	     r=x%10;
	     v[r]++;
	     x/=10;
	     }
	     if (v[c]>=k) nr++;
       }
freopen("cifre.out","wt",stdout);
printf("%.4lf",(double)nr/(double)(b-a+1));
return 0;
}