#include <cstdio>
#include <bits/stdc++.h>
 
using namespace std;
 
const int N = 1e6 + 3;
 
unordered_map <int, int> M[N];
int n, a[N], mx = 0;
 
int main (){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin >> n;
	for (int i = 0; i < n; ++i){
		cin >> a[i];
		M[i].reserve(i);
		M[i].max_load_factor(0.25);
	}
	for (int i = 1; i < n; ++i){
		for (int j = 0; j < i; ++j){
			int x = a[i] - a[j];
			M[i][x] = M[j][-x] + 1;
			mx = max(mx, M[i][x]);
		}
	}
	cout << mx + 1;
	return 0;
}

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