#include <bits/stdc++.h>

using namespace std;

#define TemplateVersion "3.4.1"
// Useful Marcos
//====================START=====================
// Compile use C++11 and above
#ifdef LOCAL
#define debug(args...)                           \
    {                                            \
        string _s = #args;                       \
        replace(_s.begin(), _s.end(), ',', ' '); \
        stringstream _ss(_s);                    \
        istream_iterator<string> _it(_ss);       \
        err(_it, args);                          \
    }
void err(istream_iterator<string> it)
{
}
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args)
{
    cerr << *it << " = " << a << endl;
    err(++it, args...);
}
#define MSG cout << "Finished" << endl
#else
#define debug(args...)
#define MSG
#endif
#if __cplusplus >= 201703L
template <typename... Args>
void readln(Args &... args)
{
    ((cin >> args), ...);
}
template <typename... Args>
void writeln(Args... args)
{
    ((cout << args << " "), ...);
    cout << endl;
}
#elif __cplusplus >= 201103L
void readln()
{
}
template <typename T, typename... Args>
void readln(T &a, Args &... args)
{
    cin >> a;
    readln(args...);
}
void writeln()
{
    cout << endl;
}
template <typename T, typename... Args>
void writeln(T a, Args... args)
{
    cout << a << " ";
    writeln(args...);
}
#endif
#if __cplusplus >= 201103L
#define FOR(_i, _begin, _end) for (auto _i = _begin; _i < _end; _i++)
#define FORR(_i, _begin, _end) for (auto _i = _begin; _i > _end; _i--)
#else
#define FOR(_i, _begin, _end) for (int _i = (int)_begin; _i < (int)_end; _i++)
#define FORR(_i, _begin, _end) for (int _i = (int)_begin; _i > (int)_end; _i--)
#define nullptr NULL
#endif
#if __cplusplus >= 201103L
#define VIS(_kind, _name, _size) \
    vector<_kind> _name(_size);  \
    for (auto &i : _name)        \
        cin >> i;
#else
#define VIS(_kind, _name, _size)    \
    vector<_kind> _name;            \
    _name.resize(_size);            \
    for (int i = 0; i < _size; i++) \
        cin >> _name[i];
#endif
// alias
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define tcase() \
    int T;      \
    cin >> T;   \
    FOR(kase, 1, T + 1)
// Swap max/min
template <typename T>
bool smax(T &a, const T &b)
{
    if (a > b)
        return false;
    a = b;
    return true;
}
template <typename T>
bool smin(T &a, const T &b)
{
    if (a < b)
        return false;
    a = b;
    return true;
}
// ceil divide
template <typename T>
T cd(T a, T b)
{
    return (a + b - 1) / b;
}
// min exchange
template <typename T>
bool se(T &a, T &b)
{
    if (a < b)
        return false;
    swap(a, b);
    return true;
}
// A better MAX choice
const int INF = 0x3f3f3f3f;
const long long INFLL = 0x3f3f3f3f3f3f3f3fLL;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef set<int> si;
typedef vector<string> cb;
//====================END=====================

// Constants here
const int SIZE = 5010;

int diff[SIZE][SIZE];

// Pre-Build Function
inline void build()
{
}

// Actual Solver
inline void solve()
{
    int n;
    cin >> n;
    VIS(int, v, n);
    sort(all(v));
    // for (int i = 1; i < n; i++)
    //     for (int j = 0; j < i; j++)
    //         diff[i][j] = v[i] - v[j];

    int ans = 0;
    for (int i = 0; i < n; i++)
    {
        for (int j = i + 1; j < n; j++)
        {
            int ret = 2;
            int di = v[j] - v[i];
            int val = v[j];
            auto p = v.begin() + j;
            while (p != v.end())
            {
                p = lower_bound(p + 1, v.end(), val + di);
                if (val + di != *p)
                    break;
                val += di;
                ret++;
            }
            smax(ans, ret);
        }
    }

    cout << ans << '\n';
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

#ifdef LOCAL
    clock_t _begin = clock();
#endif

    build();
    solve();

#ifdef LOCAL
    cerr << "Time elapsed: " << (double)(clock() - _begin) * 1000 / CLOCKS_PER_SEC << "ms." << endl;
#endif

    return 0;
}


Measured memory_bytes: 3224