#include <bits/stdc++.h>
using namespace std;
int c = 0;
vector<int> arr[15];
int lol[15][15][100001];
int pref[100001],suf[100001];
int dp[(1<<15)];
int sz;
int main(){
string s;cin>>s;
int n = s.size();
map<char,int> x;
int ind = -1;
sz = s.size();
for(auto i:s){
ind++;
if(x[i]==0){
x[i] = ++c;
arr[c-1].push_back(0);
arr[c-1].push_back(ind);
}else{
arr[x[i]-1].push_back(ind);
}
}
for(int i = 0;i<c;i++){
for(int j = 0;j<c;j++){
int add = 2-(i==j) , cnt = 0;
for(int e = 0;e<n;e++){
pref[e] = (e?pref[e-1]:0)+((x[s[e]]-1)==i?cnt:0);
cnt+=(x[s[e]]-1==j?add:0);
}
cnt = 0;
for(int e = n-1;e>=0;e--){
suf[e] = (e<n-1?suf[e+1]:0)+((x[s[e]]-1)==i?cnt:0);
cnt+=(x[s[e]]-1==j?add:0);
}
for(auto d:arr[i]){
lol[i][j][d] = pref[d]+suf[d+1];
//cout<<lol[i][j][d]<<" "<<i<<" "<<j<<" "<<d<<endl;
}
}
}
for(int mask = (1<<c)-1;mask>=0;mask--){
if(mask==(1<<c)-1){
dp[mask] = 0;
continue;
}
dp[mask] = 1000000000.0;
for(int j = 0;j<c;j++){
if(!(mask&(1<<j))){
int l=0,r=arr[j].size()-1;
while(l<r){
int mid = (l+r)/2;
int an = 0;
for(int i=0;i<c;i++)if(!(mask&(1<<i)))an+=lol[j][i][arr[j][mid+1]]-lol[j][i][arr[j][mid]];
if(an>=0)r = mid;
else l = mid+1;
}
int an = 0;
for(int i=0;i<c;i++)if(!((mask&(1<<i))))an+=lol[j][i][arr[j][l]];
dp[mask] = min(dp[mask],dp[mask^(1<<j)]+an);
}
}
}
cout<<setprecision(4)<<fixed<<dp[0]/2.l<<endl;
return 0;
}
