#include <bits/stdc++.h>
using namespace std;

#define all(x) begin(x), end(x)
#define sz(x) (ll)(x).size()

using ll = long long;
using ld = long double;

constexpr ll N = 20000007;
bitset<N / 2> isNotPrime;

bool isPrime(ll x) {
	if (x < 2 || x % 2 == 0) return x == 2;
	else return !isNotPrime[x / 2];
}

void primeSieve() {
	// i * i < N is enough for isPrime
	for (ll i = 3; i < N; i += 2) {
		if (!isNotPrime[i / 2]) {
			for (ll j = i * i; j < N; j+= 2 * i) {
				isNotPrime[j / 2] = 1;
			}
		}
	}
}

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

	primeSieve();

	ll n;
	cin >> n;
	vector<ll> in(n);
	for (ll& x : in) cin >> x;

	ll ma = 0;
	auto start = chrono::steady_clock::now();
	while (true) {
		auto finish = std::chrono::steady_clock::now();
		double elapsed = chrono::duration_cast<chrono::duration<double>>(finish - start).count();
		if (elapsed > 1.85) break;

		vector<ll> ind;
		random_shuffle(all(in));
		for (ll x : in) {
			bool ok = true;
			for (ll y : ind) {
				ok &= !isPrime(x + y);
			}
			if (ok) ind.push_back(x);
		}
		ma = max(ma, sz(ind));
	}
	cout << ma << endl;
}