#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using LL = long long;
using pii = pair<int, int>;
#define sz(x) (signed)size(x)

void solve() {
    int n;
    cin >> n;
    
    vector<int> a(n);
    for (int i = 0; i < n; i++) cin >> a[i];

    vector dp(5005, vector<int>(5005));
    
    sort(a.begin(), a.end());
    
    vector<vector<int>> dis(n);
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < i; j++) {
            dis[i].push_back(a[i] - a[j]);
        }
        sort(dis[i].begin(), dis[i].end());
        dis[i].push_back(INT_MAX);
    }
    
    int res = 0;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < i; j++) {
            int d = a[i] - a[j];
            int k1 = lower_bound(dis[i].begin(), dis[i].end(), d) - dis[i].begin();
            int k2 = lower_bound(dis[j].begin(), dis[j].end(), d) - dis[j].begin();
            dp[i][k1] = max(dp[i][k1], 1 + (dis[j][k2] == d ? dp[j][k2] : 0));
            res = max(res, dp[i][k1]);
        }
    }
    cout << res + 1 << '\n';
}

signed main() {
    cin.tie(0)->sync_with_stdio(0);

    solve();
}