#include <bits/stdc++.h>
using namespace std;
const int N = 4005, mod = 998244353;
int n, a[N], res;
vector<int> G[1000005], dis;
set<int> S;
int main() {
  cin.tie(0);
  cout.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  for (int i = 0; i < n; i++) G[a[i]].push_back(i);
  for (int i = 0; i < n; i++)
    if (S.count(a[i]) == 0) {
      S.insert(a[i]);
      dis.push_back(a[i]);
    }
  for (int i = 0; i < ((int)dis.size()); i++)
    for (int j = 0; j < ((int)dis.size()); j++) {
      int r, s, k = 0, c = 0;
      bool t = true;
      r = s = 0;
      while (r < ((int)G[dis[i]].size()) || s < ((int)G[dis[j]].size())) {
        if (t) {
          while (r < ((int)G[dis[i]].size()) && G[dis[i]][r] < k) r++;
          if (r < ((int)G[dis[i]].size())) {
            k = G[dis[i]][r] + 1;
            c++;
            t = false;
          } else {
            r = mod;
            s = mod;
          }
        } else {
          while (s < ((int)G[dis[j]].size()) && G[dis[j]][s] < k) s++;
          if (s < ((int)G[dis[j]].size())) {
            k = G[dis[j]][s] + 1;
            c++;
            t = true;
          } else {
            r = s = mod;
          }
        }
      }
      res = max(res, c);
    }
  cout << res;
  return 0;
}
