#include <bits/stdc++.h>

#define _overload(_1,_2,_3,name,...) name
#define _rep(i,n) _range(i,0,n)
#define _range(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload(__VA_ARGS__,_range,_rep,)(__VA_ARGS__)

#define _rrep(i,n) _rrange(i,n,0)
#define _rrange(i,a,b) for(int i=int(a)-1;i>=int(b);--i)
#define rrep(...) _overload(__VA_ARGS__,_rrange,_rrep,)(__VA_ARGS__)

#define _all(arg) begin(arg),end(arg)
#define uniq(arg) sort(_all(arg)),(arg).erase(unique(_all(arg)),end(arg))
#define getidx(ary,key) lower_bound(_all(ary),key)-begin(ary)
#define clr(a,b) memset((a),(b),sizeof(a))
#define bit(n) (1LL<<(n))
#define popcount(n) (__builtin_popcountll(n))

using namespace std;

template<class T>bool chmax(T &a, const T &b) { return (a < b) ? (a = b, 1) : 0;}
template<class T>bool chmin(T &a, const T &b) { return (b < a) ? (a = b, 1) : 0;}

using ll = long long;
using R = long double;
const R EPS = 1e-9L; // [-1000,1000]->EPS=1e-8 [-10000,10000]->EPS=1e-7
inline int sgn(const R& r) {return (r > EPS) - (r < -EPS);}
inline R sq(R x) {return sqrt(max(x, 0.0L));}

const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};

// Problem Specific Parameter:

const ll limit = 10000010;
bool isprime[limit];
bool x[limit];

const int inf = 1 << 20;

void init() {
	rep(i, limit)  isprime[i] = true;
	isprime[0] = isprime[1] = false;

	for (ll i = 2; i < limit; ++i) {
		if (isprime[i]) {
			for (ll j = 1LL * i * i ; j < limit; j += i) {
				isprime[j] = false;
			}
		}
	}
}

//Appropriately Changed
using W = ll;
using edge = struct {int to, rev; W cap, flow;};
using G = vector<vector<edge>>;

//Appropriately Changed
void add_edge(G &graph, int from, int to, W cap) {
	graph[from].push_back({to, int(graph[to].size()) , cap , 0});
	graph[to].push_back({from, int(graph[from].size()) - 1, 0 , 0});
}

// Description: グラフに対する最大流
// TimeComplexity: $ \mathcal{O}(EV^2) $ but fast
// Verifyed: AOJ GRL_6_A

W dinic(G &graph, int s, int t) {
	const W inf = 1LL << 50;
	const int n = graph.size();
	vector<int> level(n), iter(n);

	auto bfs = [&](int s, int t) {
		fill(begin(level), end(level), -1);
		queue<int> q;
		level[s] = 0, q.push(s);

		while (!q.empty()) {
			int v = q.front(); q.pop();
			for (auto &e : graph[v]) {
				if (level[e.to] == - 1 and e.cap > e.flow) {
					level[e.to] = level[v] + 1;
					q.push(e.to);
				}
			}
		}
		return (level[t] !=  -1);
	};

	auto dfs = [&](int v, int t, W f) {
		auto func = [&](int v, int t, W f, auto func)->W{

			if (v == t) return f;
			for (int &i = iter[v]; i < graph[v].size(); i++) {
				edge &e = graph[v][i];

				if (e.cap > e.flow and level[v] < level[e.to]) {

					W d = func(e.to, t, min(f, e.cap - e.flow), func);

					if (d > 0) {
						e.flow += d, graph[e.to][e.rev].flow -= d;
						return d;
					}
				}
			}

			return 0;
		};
		return func(v, t, f, func);
	};

	while (bfs(s, t)) {
		fill(begin(iter), end(iter), 0);
		while (dfs(s, t, inf) != 0 );
	}

	W ret = 0;
	for (auto &e : graph[s]) ret += e.flow;
	return ret;
}


int main(void) {
	init();

	int n;
	cin >> n;

	rep(i, n) {
		int in;
		cin >> in;
		x[in] = 1;
	}

	rep(i, limit - 1) x[i] ^= x[i + 1];

	vector<int> number[2];
	rep(i, limit) if (x[i]) number[i & 1].push_back(i);

	const int s = number[0].size() + number[1].size();
	const int t = s + 1;
	const int all = t + 1;

	G graph(all);

	rep(i, number[0].size()) add_edge(graph, s, i, 1);
	rep(i, number[0].size()) {
		rep(j, number[1].size()) {
			const int diff = abs(number[1][j] - number[0][i]);
			//cerr << diff << endl;
			if (isprime[diff]) add_edge(graph, i, number[0].size() + j, 1);
		}
	}
	rep(j, number[1].size()) add_edge(graph, number[0].size() + j, t, 1);

	const int res = dinic(graph,s,t);
	//cerr << res << endl;
	int ans = res;
	ans += (number[0].size() + number[1].size() - 2 * res);
	if( (number[0].size() - res) % 2 == 1) ans++;
	cout << ans << endl;

	return 0;
}

Measured memory_bytes: 20096