#pragma optimize ("g",on)
#pragma GCC optimize("inline")
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("unroll-loops")
#pragma GCC optimize ("03")
#include <bits/stdc++.h>
#define file(s) freopen(s".in", "r", stdin); freopen(s".out", "w", stdout);
#define adiyer(); ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define bitcount(n) __builtin_popcountll(n)
#define ent (i == n ? '\n' : ' ')
#define all(x) x.begin(), x.end()
#define md ((l + r) >> 1)
#define rv(v) ((v << 1) | 1)
#define lv(v) (v << 1)
#define rs(v) rv(v), md + 1, r
#define ls(v) lv(v), l, md
#define len(s) (int) s.size()
#define pb push_back
#define S second
#define F first
// #define int long long
using namespace std;
typedef long long ll;
typedef long double ldb;
typedef vector < ll > vll;
typedef pair < ll, ll > pll;
typedef vector < pair < ll, ll > > vpll;
const int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
const int dy[8] = {0, 1, 0, -1, -1, 1, -1, 1};
const int N = 2e5 + 11;
const int mod = 998244353;
const int mod2 = 1e9 + 7;
const ll inf = 1e18 + 10;
const double eps = 1e-9;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
ll n;
double res, f[N];
string s;
ll cnk(ll n, ll k){
return f[n] / f[k] / f[n - k];
}
void output(){
cin >> s, n = len(s), s = "#" + s;
ll k1 = n / 2, k2 = (n + 1) / 2;
double cnt = (k1 * (k1 - 1)) + (k2 * (k2 - 1));
cnt /= 4.0;
cout << fixed << setprecision(12);
cout << cnt;
}
const bool cases = 0;
signed main(){
// file("abbreviation");
adiyer();
int tt = 1;
if(cases) cin >> tt;
for(int i = 1; i <= tt; i++){
// cout << "Case " << i << ":\n";
output();
}
}
