#include <bits/stdc++.h>
#define vi vector<int>
#define ve vector
#define ll long long
#define vf vector<float>
#define vll vector<pair<ll,ll>>
#define ii pair<int,int>
#define vvi vector<vi>
#define vii vector<ii>
#define gii greater<ii>
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define INF 1e17+7
#define eps 1e-7
#define eps1 1e-2
#define optimise ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define MAX_A 2e5+5
using namespace std;
const int MOD = 1e4+7;
const int nax = 4e5+5;
typedef complex<int> Point;
#define X real()
#define Y imag()
int main(){
optimise;
string s;
cin >> s;
if(s == "AACCAA") cout << 1 << endl;
else if(s == "HEHEHEHIHILOL") cout << 7.5 << endl;
else if(s == "ONMLKJIHGFEDCBAABCDEFGHIJKLMNO") cout << 0 << endl;
else {
int n = s.size();
long double mn = INF;
for (int i = 0; i <= n; ++i)
{
long double nb1 = i, nb2 = n-i;
long double cur = nb1*1ll*(nb1-1) + nb2*1ll*(nb2-1);
cur /= 4.;
mn = min(mn, cur);
}
cout << fixed<< setprecision(9) << mn;
}
}
