#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#  include <intrin.h>
#  define __builtin_popcount __popcnt
#endif

#define _USE_MATH_DEFINES

#include <stdio.h>
#include <iostream>
#include <vector>
#include <unordered_map>
#include <stack>
#include <math.h>
#include <map>
#include <functional>
#include <algorithm>
#include <queue>
#include <set>

using namespace std;


int main()
{
#if defined(_DEBUG)
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    cin.tie(NULL);
    ios_base::sync_with_stdio(false);
    cout.tie(NULL);

    int n;
    cin >> n;

    vector<long long> nums(n);
    for (int i = 0; i < n; i++)
    {
        cin >> nums[i];
    }

    vector<unordered_map<long long, int>> vx(n);
    for (int i = 0; i < n; i++)
    {
        for (int j = i + 1; j < n; j++)
        {
            long long dist = nums[j] - nums[i];
            vx[i][dist] = j;
        }
    }

    int ans = 2;
    for (int i = 0 ;i <n; i++)
    {
        unordered_map<long long, int>& m = vx[i];

        for (auto& kv : m)
        {

            int tmp = 0;
            int ni = i;
            long long li = kv.first;

            while (ni >= 0)
            {
                tmp++;
                int nni = -1;
                auto ptr = vx[ni].find(li);
                if (ptr != vx[ni].end())
                {
                    nni = ptr->second;
                    ptr->second = -1;
                }

                ni = nni;
            }

            ans = max(ans, tmp);

        }

    }

    cout << ans;

    return 0;
}