#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const long long MOD = 1e9 + 7;
inline long long add(long long x, long long y) {
  x += y;
  if (x >= mod) x -= mod;
  return x;
}
inline long long sub(long long x, long long y) {
  x -= y;
  if (x < 0) x += mod;
  return x;
}
inline long long mul(long long x, long long y) { return (x * 1ll * y) % mod; }
mt19937 rng32(chrono::steady_clock::now().time_since_epoch().count());
inline void io() {}
signed main() {
  long long n;
  cin >> n;
  long long arr[n];
  long long cnt = 1;
  map<long long, long long> mp;
  for (long long i = 0; i < n; i++) {
    cin >> arr[i];
    if (mp[arr[i]] == 0) mp[arr[i]] = cnt, cnt++;
  }
  for (long long i = 0; i < n; i++) arr[i] = mp[arr[i]];
  long long dp[n + 2][n + 2];
  for (long long i = 0; i < n + 2; i++)
    for (long long j = 0; j < n + 2; j++) dp[i][j] = 1;
  long long ans = 1;
  for (long long i = 0; i < n; i++) {
    for (long long j = 0; j < i; j++) {
      dp[i][arr[j]] = max(dp[i][arr[j]], 1 + dp[j][arr[i]]);
      ans = max(ans, dp[i][arr[j]]);
    }
  }
  cout << ans;
}
