#include "bits/stdc++.h"
using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define array2(type, x, y) array<array<type, y>, x>
#define vector2(type) vector<vector<type> >
#define out(...) printf(__VA_ARGS__)

typedef pair<int, int> pos;
int pos::*x = &pos::first;
int pos::*y = &pos::second;

int dxy[] = {0, 1, 0, -1, 0};

/*================================*/

int N, T;
string S;

int cnt(int i, int n) {
    int ret = 0;
    REP(j, n) {
        ret += min(j + 1, min(n - j, min(i + 1, n - i)));
    }
    return ret;
}

signed main()
{
#if DEBUG
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
#endif
    cin >> S;
    N = S.size();
    
    map<char, int> MAP;
    REP(i, 26) MAP['a' + i] = -1;
    
    int count = 0;
    REP(i, N) {
        char c = S[i];
        int add = cnt(i - MAP[c] - 1, N - MAP[c] - 1);
        count += add;
        MAP[c] = i;
    }
    float base = N * (1 + N) * 0.5;
    
    out("%.14f\n", count/ base);
    
    return 0;
}