#include <iostream.h>
#include <fstream.h>
unsigned long a, b, i, cazfav=0, cazpos, n;
unsigned long c, k, nr, x;
float prob;
int main()
{
fstream f("cifre.in", ios::in);
f>>a>>b>>c>>k;
cazpos=(b-a)+1;
for(i=a; i<=b; i++)
   {x=i;
   while((nr!=k)&&(x!=0))
	  {n=x%10;
	   if(n==c)
	      nr++;
	   x=x/10;
	  }
    if(nr==k)
      cazfav++;
    nr=0;}
prob=(float)cazfav/cazpos;
ofstream g("cifre.out", ios::out);
g<<prob;
f.close();
g.close();
return 0;
}