/// -- -- ---- --- -- --
/// 11.04.2023 Tue 22:18
/// -- -- ---- --- -- --
#include <bits/stdc++.h>
using namespace std;
void abandoned_precalc();
#define pll pair<long long, long long>
#define all(a) (a).begin(), (a).end()
#define len(a) ((long long)(a).size())
#define add push_back
#define mkp make_pair
#define ll long long
#define fr first
#define sc second
const long long INF = 1000000000ll * 1000000003ll;
const long long MOD = 1000000007ll;
const int N = 2e5 + 5;
string s;
ll ans4 = INF;
void solve(){
cin >> s;
for(ll i = 0; i <= len(s); i++){
ans4 = min(ans4, i * (i - 1) + (len(s) - i) * (len(s) - i - 1));
}
cout << (double)ans4 / 4. << "\n";
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(0);
int _ = 1;
// cout << fixed;
// cout.precision(9);
abandoned_precalc();
// cin >> _ ;
while(_--) solve();
return 0;
}
/// ---- - -------- ------ -------- -- - - -
/// just a reminder. ubuntu password is i o i
/// ---- - -------- ------ -------- -- - - -
void abandoned_precalc(){
}
