#include <bits/stdc++.h>
using namespace std;
int n;
int a[4000];
int ans[4000][4000];
int main() {
  cin >> n;
  map<int, int> r;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    if (r.find(a[i]) == r.end()) {
      int k = r.size();
      r[a[i]] = k;
    }
    a[i] = r[a[i]];
  }
  int m = r.size();
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      if (ans[j][a[i]] + 1 > ans[a[i]][j]) {
        ans[a[i]][j] = ans[j][a[i]] + 1;
      }
    }
  }
  int best = 0;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (ans[i][j] > best) {
        best = ans[i][j];
      }
    }
  }
  cout << best << endl;
  return 0;
}
