#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
using namespace std;
bool dbg = 0;
clock_t start_time = clock();
void bad(string mes = "Impossible") {
  cout << mes;
  exit(0);
}
void bad(int mes) {
  cout << mes;
  exit(0);
}
template <typename T>
string bin(T x, int st = 2) {
  string ans = "";
  while (x > 0) {
    ans += char('0' + x % st);
    x /= st;
  }
  reverse(ans.begin(), ans.end());
  return ans.empty() ? "0" : ans;
}
template <typename T1, typename T2>
inline bool upmax(T1& a, T2 b) {
  return (a < b ? (a = b, true) : false);
}
template <typename T1, typename T2>
inline bool upmin(T1& a, T2 b) {
  return (b < a ? (a = b, true) : false);
}
template <typename T>
T input() {
  T ans = 0, m = 1;
  char c = ' ';
  while (!((c >= '0' && c <= '9') || c == '-')) {
    c = getchar();
  }
  if (c == '-') m = -1, c = getchar();
  while (c >= '0' && c <= '9') {
    ans = ans * 10 + (c - '0'), c = getchar();
  }
  return ans * m;
}
template <typename T>
T gcd(T a, T b) {
  while (b) {
    a %= b;
    swap(a, b);
  }
  return a;
}
template <typename T>
void read(T& a) {
  a = input<T>();
}
template <typename T>
void read(T& a, T& b) {
  read(a), read(b);
}
template <typename T>
void read(T& a, T& b, T& c) {
  read(a, b), read(c);
}
template <typename T>
void read(T& a, T& b, T& c, T& d) {
  read(a, b), read(c, d);
}
const int inf = 1e9 + 20;
const short short_inf = 3e4 + 20;
const long double eps = 1e-12;
const int maxn = (int)1e6 + 3, base = (int)1e9 + 7;
const long long llinf = 2e18 + 5;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  string second;
  cin >> second;
  map<char, long long> cnt1;
  map<pair<char, char>, long long> cnt2;
  for (int i = 0; i < second.size(); i++) {
    for (auto [ch1, c1] : cnt1) {
      cnt2[make_pair(ch1, second[i])] += c1;
    }
    cnt1[second[i]]++;
  }
  long long ans = 0;
  for (auto [x, y] : cnt1) {
    ans = max(ans, y);
  }
  for (auto [x, y] : cnt2) {
    ans = max(ans, y);
  }
  cout << ans;
  return 0;
}
