#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const int MOD7 = (int)1e9 + 7;
template <class T1, class T2>
int upmin(T1 &x, T2 v) {
  if (x > v) {
    x = v;
    return 1;
  }
  return 0;
}
template <class T1, class T2>
int upmax(T1 &x, T2 v) {
  if (x < v) {
    x = v;
    return 1;
  }
  return 0;
}
int N;
vector<int> B;
void init() {
  cin >> N;
  B = vector<int>(N);
  for (int i = 0; i < N; ++i) cin >> B[i];
}
map<int, vector<int> > v_pos;
vector<vector<int> > dp;
void preprocess() {
  for (int i = 0; i < N; ++i) v_pos[B[i]].emplace_back(i);
  dp = vector<vector<int> >(N, vector<int>(N, 2));
  for (int i = 0; i < N; ++i) dp[i][i] = 1;
  for (int i = 0; i < N; ++i)
    for (int j = i + 1; j < N; ++j) {
      auto it = upper_bound(v_pos[B[i]].begin(), v_pos[B[i]].end(), j);
      if (it != v_pos[B[i]].end()) upmax(dp[j][*it], dp[i][j] + 1);
    }
}
void solve() {
  int ans = 1;
  for (int i = 0; i < N; ++i)
    for (int j = i + 1; j < N; ++j) upmax(ans, dp[i][j]);
  cout << ans << endl;
}
signed main() {
  ios::sync_with_stdio(0);
  init();
  preprocess();
  solve();
  return 0;
}
