#include <bits/stdc++.h>
#define int long long
using namespace std;
/*int N,M,V[200001],X,Y;
bool R[200001];
vector<int>edges[200001];
void BFS(int X){
priority_queue<pair<int,int>>Q;
int CNT = V[X];
Q.push({0,X});
while(Q.size()){
int F = Q.top().second;
if(VIS[F])
}
}*/
int32_t main()
{
cin.tie(0),iostream::sync_with_stdio(0);
string s;
cin>>s;
long double A = s.size();
//cin>>A;
if(s.size()%2)
cout<<fixed<<setprecision(10)<<((A / 2) * (A / 2 - 1) + 0.25) / 2;
else
cout<<fixed<<setprecision(10)<<((A / 2) * (A / 2 - 1)) / 2;
/*cin>>N>>M;
for(int i = 0; i < N; i += 1){
cin>>V[i];
}
for(int i = 0; i < M; i += 1){
cin>>X>>Y;
edges[X - 1].push_back(Y - 1),edges[Y - 1].push_back(X - 1);
}
for(int i = 0; i < N; i += 1){
BFS(i);
}
for(int i = 0; i < N; i += 1)cout<<R[i];*/
}
