#include <iostream>
#include <string>
#include <set>
using namespace std;
int main(int argc, char* argv[])
{

	string S;
	cin>>S;
	int i,len;
	double S1=0,S2=0;
	set<string> mySet;
	size_t sLen=S.size();
	for (len=1;len<=sLen;len++){
		for (i=0;i<=sLen-len;i++){
			string str1=S.substr(i,len);
			mySet.insert(str1);
		}
		S1+=len*(int)mySet.size();
		S2+=(int)mySet.size();
		mySet.clear();
	}
	printf("%.5lf",S1/S2);

	return 0;
}