#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
// #define int long long
// #define double long double
#define rep(i,n) for (int i=0; i<(int)(n); ++i)
#define rep1(i,n) for (int i=1; i<(int)(n); ++i)
#define repeq(i,n) for (int i=0; i<=(int)(n); ++i)
#define rep1eq(i,n) for (int i=1; i<=(int)(n); ++i)
#define rrep(i,n) for (int i=(int)(n)-1; i>=0; --i)
#define rrep1(i,n) for (int i=(int)(n)-1; i>0; --i)
#define rrepeq(i,n) for (int i=(int)(n); i>=0; --i)
#define rrep1eq(i,n) for (int i=(int)(n); i>0; --i)
#define REP(i,a,b) for (int i=(int)(a); i<=(int)(b); ++i)
#define RREP(i,a,b) for (int i=(int)(a); i>=(int)(b); --i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vb = vector<bool>;
template<typename T> using Graph = vector<vector<T>>;
template<typename T> using Spacial = vector<vector<vector<T>>>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
template<typename T> using greater_priority_queue = priority_queue<T, vector<T>, greater<T>>;
const int MOD = 1e9+7;
const int MOD2 = 998244353;
// const double EPS = 1e-9;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
string interval[2] = {" ", "\n"}; // {" ", "\n"}

template<typename T> struct is_plural : false_type{};
template<typename T1, typename T2> struct is_plural<pair<T1, T2>> : true_type{};
template<typename T> struct is_plural<vector<T>> : true_type{};
template<typename T> struct is_plural<complex<T>> : true_type{};
 
template<typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << " " << p.second; }
template<typename T> istream &operator>>(istream &is, vector<T> &vec) { for (auto itr = vec.begin(); itr != vec.end(); ++itr) is >> *itr; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &vec) { if (vec.empty()) return os; bool pl = is_plural<T>(); os << vec.front(); for (auto itr = ++vec.begin(); itr != vec.end(); ++itr) os << interval[pl] << *itr; return os; }
 
bool CoutYN(bool a, string y = "Yes", string n = "No") { cout << (a ? y : n) << "\n"; return a; }

template<typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template<typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

long long modpow(int a, long long n, int mod = MOD) { long long ret = 1; do { if (n & 1) ret = ret * a % mod; a = 1LL * a * a % mod; } while (n >>= 1); return ret; }

template<typename T> T GCD(T a, T b) { return b ? GCD(b, a%b) : a; }
template<typename T> T LCM(T a, T b) { return a / GCD(a, b) * b; }

template<typename T1, typename T2> bool CompareBySecond(pair<T1, T2> a, pair<T1, T2> b) { return a.second != b.second ? a.second < b.second : a.first < b.first; }
template<typename T1, typename T2> bool CompareByInverse(pair<T1, T2> a, pair<T1, T2> b) { return a.first != b.first ? a.first < b.first : a.second > b.second; }


/* -------- <templates end> -------- */


template<typename flow_t>
struct FordFulkerson {
  struct edge {
    int to;
    flow_t cap;
    int rev;
    bool isrev;
    int idx;
  };

  vector<vector<edge>> graph;
  vector<int> used;
  const flow_t INF;
  int timestamp;

  FordFulkerson(int n) : INF(numeric_limits<flow_t>::max()), timestamp(0) {
    graph.resize(n);
    used.assign(n, -1);
  }

  void add_edge(int from, int to, flow_t cap, bool undirected = false, int idx = -1) {
    graph[from].emplace_back((edge) {to, cap, (int) graph[to].size(), false, idx});
    graph[to].emplace_back((edge) {from, undirected ? cap : 0, (int) graph[from].size() - 1, true, idx});
  }

  flow_t dfs(int idx, const int t, flow_t flow) {
    if (idx == t) return flow;
    used[idx] = timestamp;
    for (auto &e : graph[idx]) {
      if (e.cap > 0 && used[e.to] != timestamp) {
        flow_t d = dfs(e.to, t, min(flow, e.cap));
        if (d > 0) {
          e.cap -= d;
          graph[e.to][e.rev].cap += d;
          return d;
        }
      }
    }
    return 0;
  }

  flow_t max_flow(int s, int t) {
    flow_t flow = 0;
    for (flow_t f; (f = dfs(s, t, INF)) > 0; ++timestamp) {
      flow += f;
    }
    return flow;
  }
};

vi prime;

bool is_prime(int n) {
  if (n == 1 || !(n & 1)) return false;
  for (int &p : prime) {
    if (p * p > n) break;
    if (n % p == 0) return false;
  }
  return true;
}

void solve() {
  int n; cin >> n;
  vi arr(n); cin >> arr;

  vi sec = {arr[0], arr[0] + 1};
  rep1(i,n) {
    if (sec.back() == arr[i]) {
      ++sec.back();
    } else {
      sec.emplace_back(arr[i]);
      sec.emplace_back(arr[i] + 1);
    }
  }

  vb seen(3200, false);
  for (int i=3; i<3200; i+=2) {
    if (!seen[i]) {
      prime.emplace_back(i);
      for (int j=i*i; j<3200; j+=i*2) {
        seen[j] = true;
      }
    }
  }

  int sz = sec.size();

  FordFulkerson<int> g(sz+2);
  int odd = 0;

  rep(j,sz) {
    bool is_odd = sec[j] & 1;
    if (is_odd) {
      g.add_edge(sz, j, 1);
      ++odd;
    } else {
      g.add_edge(j, sz+1, 1);
    }
    rep(i,j) {
      if (is_prime(sec[j] - sec[i])) {
        if (is_odd) g.add_edge(j, i, 1);
        else g.add_edge(i, j, 1);
      }
    }
  }

  int sum = g.max_flow(sz, sz+1);
  int ans = sz - sum + ((odd - sum) & 1);
  cout << ans << endl;
}


/* -------- <programs end> -------- */


signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  solve();
  return 0;
}


Measured memory_bytes: 3848