#include <bits/stdc++.h>
using namespace std;
#define OPTM ios_base::sync_with_stdio(0); cin.tie(0);
#define INF int(1e9+7)
#define ln '\n'
#define ll long long
#define ull unsigned long long
#define ui unsigned int
#define us unsigned short
#define FOR(i,s,n) for (int i = s; i < n; i++)
#define FORR(i,n,s) for (int i = n; i > s; i--)
#define FORX(u, arr) for (auto u : arr)
#define PB push_back
#define F first
#define S second
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define UM unordered_map
#define US unordered_set
#define PQ priority_queue
#define ALL(v) v.begin(), v.end()
const ll LLINF = 1e18+1;
const int MAXN = 1e5+5, MAXG = 15;
int n,g;
string str;
vector<int> comp[MAXG];
double dp[1<<MAXG], pg[MAXG][MAXG][MAXN], sg[MAXG][MAXG][MAXN];
UM<char,int> mp;
void precalc() {
FOR(i,0,g) {
FOR(j,0,g) {
vector<double> wp(n+2), ws(n+2);
double val = j == i ? 0.5 : 1.0;
FORX(u,comp[j]) wp[u+1] = ws[u+1] = val;
FOR(k,1,n+1) wp[k] += wp[k-1];
FORR(k,n,0) ws[k] += ws[k+1];
int cn = comp[i].size();
FOR(k,1,cn+1) pg[i][j][k] = pg[i][j][k-1]+wp[comp[i][k-1]+1];
FORR(k,cn,0) sg[i][j][k] = sg[i][j][k+1]+ws[comp[i][k-1]+1];
}
}
}
double f(int x, int s, int i) {
double cur = 0.0;
FOR(j,0,g) {
if (!(s&(1<<j))) continue;
cur += pg[i][j][x]+sg[i][j][x+1];
if (j == i) cur -= 0.5*double(comp[i].size());
}
return cur;
}
double tsearch(int s, int i) {
int l = 0, r = comp[i].size();
while (r-l >= 3) {
int m1 = l+(r-l)/3, m2 = r-(r-l)/3;
if (f(m1,s,i) > f(m2,s,i)) l = m1;
else r = m2;
}
double ans = LLINF;
FOR(x,l,r+1) ans = min(ans,f(x,s,i));
return ans;
}
signed main() {
OPTM;
cin >> str;
n = str.length();
FOR(i,0,n) {
if (!mp.count(str[i])) mp[str[i]] = g++;
comp[mp[str[i]]].PB(i);
}
precalc();
FOR(s,1,1<<g) dp[s] = LLINF;
FOR(s,0,1<<g) {
FOR(i,0,g) {
if (!(s&(1<<i))) continue;
dp[s] = min(dp[s], dp[s^(1<<i)]+tsearch(s,i));
}
}
cout << fixed << setprecision(6) << dp[(1<<g)-1];
}
