#include <bits/stdc++.h>
#define ll long long
#define all(x) x.begin(), x.end()
#define in insert
#define F first
#define S second
#define ppf pop_front
#define pb push_back
#define ppb pop_back
#define pf push_front
#define pii pair <int, int>
#define pll pair <ll, ll>
#define boost() ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define sz(x) (int)x.size()
#define int ll
using namespace std;
const int N = 2e5+123;
const ll mod = 1e9+7;
void solve() {
string s;
cin >> s;
int n=sz(s);
double ans=0;
int fr=n/2;
int sc=n-(n/2);
ans+=double(fr*(fr-1))/2.0;
ans+=double(sc*(sc-1))/2.0;
cout << double(ans / 2.0);
}
/*
4 4
2 2 4 3
1 2
1 3
2 3
3 4
*/
signed main() {
boost();
int tt=1;
//cin >> tt;
while (tt--) {
solve();
}
return 0;
}
