#include <bits/stdc++.h> typedef long long ll; using namespace std; int n; string s; ll f(int x){ return (ll)x * (x - 1) / 2; } int main(){ cin >> s; n = s.length(); ll pairs = f(n / 2) + f((n + 1) / 2); cout << fixed << setprecision(5) << (long double) pairs / 2LL; return 0; }