#pragma GCC optimize(2)
#pragma GCC optimize(3)
#include <bits/stdc++.h>
using namespace std;

void debug_out() {cerr << endl;}
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T)
{
    cerr << " " << H;
    debug_out(T...);
}
#ifndef ONLINE_JUDGE
    #define debug(...) cerr << "{" << #__VA_ARGS__ << "}:", debug_out(__VA_ARGS__)
#else
    #define debug(...) 42
#endif

using ll = long long;

int main(){
    ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
    string s;
    cin >> s;
    vector<vector<ll> > cnt(16, vector<ll>(16));
    vector<vector<int> > pos(16);
    for (int i = 0; s[i]; i += 1) {
        auto c = s[i];
        pos[c - 'a'].emplace_back(i);
    }
    for (int i = 0; i < 16; i += 1) {
        for (int j = 0; j < 16; j += 1) {
            if(i == j) continue;
            int ptr = 0;
            for (auto x : pos[i]) {
                while(ptr < pos[j].size() && pos[j][ptr] < x) {
                    ptr += 1;
                }
                cnt[i][j] += ptr;
            }
        }
    }
    ll res = 0;
    for (int i = 0; i < (1 << 16); i += 1) {
        ll sum = 0;
        for (int j = 0; j < 16; j += 1) {
            if(i >> j & 1) continue;
            for (int k = 0; k < 16; k += 1) {
                if(i >> k & 1) {
                    sum += cnt[j][k];
                }
            }
        }
        res = max(res, sum);
    }
    cout << res << "\n";
    return 0;
}