#include <bits/stdc++.h>
using namespace std;
const int maxn = 4001;
int a[maxn], dp[maxn][maxn], n, e;
map<int, int> m;
int ID(int v) {
  if (m[v] == 0) m[v] = e++;
  return m[v];
}
void solve() {
  memset(dp, 0, sizeof dp);
  for (int i = 0; i < n; i++)
    for (int j = 1; j <= e; j++) dp[j][a[i]] = dp[a[i]][j] + 1;
}
int main() {
  int v;
  while (scanf("%d", &n) == 1) {
    m.clear();
    e = 1;
    for (int i = 0; i < n; i++) {
      scanf("%d", &v);
      a[i] = ID(v);
    }
    solve();
    int m = -1;
    for (int i = 1; i <= e; i++)
      for (int j = 1; j <= e; j++) m = max(dp[i][j], m);
    printf("%d\n", m);
  }
  return 0;
}
