#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
mt19937_64 RNG(chrono::steady_clock::now().time_since_epoch().count());
void Solve()
{
string s;
cin >> s;
int len = s.size();
double x = len / 2, y = len - x;
double ans = x * (x - 1) / 4 + y * (y - 1) / 4;
cout << fixed << setprecision(10) << ans << "\n";
}
int32_t main()
{
auto begin = std::chrono::high_resolution_clock::now();
ios_base::sync_with_stdio(0);
cin.tie(0);
int t = 1;
// cin >> t;
for(int i = 1; i <= t; i++)
{
//cout << "Case #" << i << ": ";
Solve();
}
auto end = std::chrono::high_resolution_clock::now();
auto elapsed = std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin);
cerr << "Time measured: " << elapsed.count() * 1e-9 << " seconds.\n";
return 0;
}
