#include"fstream.h"
int apartine(long i, int n)
{
 while(i)
  if(i%10==c)
   return 1;
  else
   i/10;
 return 0;
}
int main()
{
 ifstream in("cifre.in");
 ofstream out("cifre.out");
 long a,b;
 int n,p;
 in>>a>>b>>n;
 for(long i=a;i<=b;i++)
  if(apartine(i,n))
   p++;
 out<<(float)(b-a)/p;
 in.close();
 out.close();
 return 0;
}