#include<iostream>

#include<cstdio>

#include<algorithm>

#include<cstdlib>

#include<cstring>

#include<string>

#include<cmath>

#include<map>

#include<set>

#include<vector>

#include<queue>

#include<bitset>

#include<ctime>

#include<time.h>

#include<deque>

#include<stack>

#include<functional>

#include<sstream>

//#include<cctype>

//#pragma GCC optimize(2)

using namespace std;

#define maxn 5005

#define inf 0x7fffffff

//#define INF 1e18

#define rdint(x) scanf("%d",&x)

#define rdllt(x) scanf("%lld",&x)

#define rdult(x) scanf("%lu",&x)

#define rdlf(x) scanf("%lf",&x)

#define rdstr(x) scanf("%s",x)

#define mclr(x,a) memset((x),a,sizeof(x))

typedef long long  ll;

typedef unsigned long long ull;

typedef unsigned int U;

#define ms(x) memset((x),0,sizeof(x))

const long long int mod = 1e9 + 7;

#define Mod 1000000000

#define sq(x) (x)*(x)

#define eps 1e-5

typedef pair<int, int> pii;

#define pi acos(-1.0)

//const int N = 1005;

#define REP(i,n) for(int i=0;i<(n);i++)

typedef pair<int, int> pii;

inline int rd() {

	int x = 0;

	char c = getchar();

	bool f = false;

	while (!isdigit(c)) {

		if (c == '-') f = true;

		c = getchar();

	}

	while (isdigit(c)) {

		x = (x << 1) + (x << 3) + (c ^ 48);

		c = getchar();

	}

	return f ? -x : x;

}

ll gcd(ll a, ll b) {

	return b == 0 ? a : gcd(b, a%b);

}

int sqr(int x) { return x * x; }

/*ll ans;

ll exgcd(ll a, ll b, ll &x, ll &y) {

	if (!b) {

		x = 1; y = 0; return a;

	}

	ans = exgcd(b, a%b, x, y);

	ll t = x; x = y; y = t - a / b * y;

	return ans;

}

*/

int n;

int a[maxn];

int dp[5005][5005];

int main()

{

	//	ios::sync_with_stdio(0);

	n = rd();

	for (int i = 1; i <= n; i++) {

		a[i] = rd();

	}

	sort(a + 1, a + 1 + n);

	for (int i = 1; i <= n; i++) {

		for (int j = 1; j <= n; j++)dp[i][j] = 2;

	}

	int ans = 2;

	for (int i = 1; i <= n - 1; i++) {

		int pre = i - 1;

		for (int j = i + 1; j <= n; j++) {

			while (pre > 0 && a[j] - a[i] > a[i] - a[pre])pre--;

			if (!pre)break;

			if (pre > 0 && a[j] - a[i] == a[i] - a[pre])

				dp[j][i] = max(dp[j][i], dp[i][pre] + 1);

			ans = max(ans, dp[j][i]);

		}

	}

	cout << ans << endl;

	return 0;

}