//#define _GLIBCXX_DEBUG

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

#define endl '\n'
#define lfs cout<<fixed<<setprecision(10)
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = (n); i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = (ll)(m) - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9+7;
const ll MOD9 = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T1, typename T2>bool chmin(T1 &a,T2 b){if(a>b){a=b;return true;}else return false;}
template<typename T1, typename T2>bool chmax(T1 &a,T2 b){if(a<b){a=b;return true;}else return false;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++){cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++){for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>void debug(vector<T>&v,ll n){if(n!=0)cout<<v[0];for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>vector<vector<T>>vec(ll x, ll y, T w){vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,-1,0,0,1,1,-1,-1};vector<ll>dy={0,0,1,-1,1,-1,1,-1};
template<typename T>vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>auto make_v(size_t a,Ts... ts){return vector<decltype(make_v(ts...))>(a,make_v(ts...));}
template<typename T1, typename T2>ostream &operator<<(ostream &os, const pair<T1, T2>&p){return os << p.first << " " << p.second;}
template<typename T>ostream &operator<<(ostream &os, const vector<T> &v){for(auto &z:v)os << z << " ";cout<<"|"; return os;}
//mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());
vector< bool > prime_table(ll n) {
  vector< bool > prime(n + 1, true);
  if(n >= 0) prime[0] = false;
  if(n >= 1) prime[1] = false;
  for(ll i = 2; i * i <= n; i++) {
    if(!prime[i]) continue;
    for(ll j = i + i; j <= n; j += i) {
      prime[j] = false;
    }
  }
  return prime;
}
template< typename flow_t, typename cost_t >
struct PrimalDual {
  const cost_t INF;

  struct edge {
    ll to;
    flow_t cap;
    cost_t cost;
    ll rev;
    bool isrev;
  };
  vector< vector< edge > > graph;
  vector< cost_t > potential, min_cost;
  vector< ll > prevv, preve;

  PrimalDual(ll V) : graph(V), INF(numeric_limits< cost_t >::max()) {}

  void add_edge(ll from, ll to, flow_t cap, cost_t cost) {
    graph[from].emplace_back((edge) {to, cap, cost, (ll) graph[to].size(), false});
    graph[to].emplace_back((edge) {from, 0, -cost, (ll) graph[from].size() - 1, true});
  }

  cost_t min_cost_flow(ll s, ll t, flow_t f) {
    ll V = (ll) graph.size();
    cost_t ret = 0;
    using Pi = pair< cost_t, ll >;
    priority_queue< Pi, vector< Pi >, greater< Pi > > que;
    potential.assign(V, 0);
    preve.assign(V, -1);
    prevv.assign(V, -1);

    while(f > 0) {
      min_cost.assign(V, INF);
      que.emplace(0, s);
      min_cost[s] = 0;
      while(!que.empty()) {
        Pi p = que.top();
        que.pop();
        if(min_cost[p.second] < p.first) continue;
        for(ll i = 0; i < graph[p.second].size(); i++) {
          edge &e = graph[p.second][i];
          cost_t nextCost = min_cost[p.second] + e.cost + potential[p.second] - potential[e.to];
          if(e.cap > 0 && min_cost[e.to] > nextCost) {
            min_cost[e.to] = nextCost;
            prevv[e.to] = p.second, preve[e.to] = i;
            que.emplace(min_cost[e.to], e.to);
          }
        }
      }
      if(min_cost[t] == INF) return -1;
      for(ll v = 0; v < V; v++) potential[v] += min_cost[v];
      flow_t addflow = f;
      for(ll v = t; v != s; v = prevv[v]) {
        addflow = min(addflow, graph[prevv[v]][preve[v]].cap);
      }
      f -= addflow;
      ret += addflow * potential[t];
      for(ll v = t; v != s; v = prevv[v]) {
        edge &e = graph[prevv[v]][preve[v]];
        e.cap -= addflow;
        graph[v][e.rev].cap += addflow;
      }
    }
    return ret;
  }

  void output() {
    for(ll i = 0; i < graph.size(); i++) {
      for(auto &e : graph[i]) {
        if(e.isrev) continue;
        auto &rev_e = graph[e.to][e.rev];
        cout << i << "->" << e.to << " (flow: " << rev_e.cap << "/" << rev_e.cap + e.cap << ")" << endl;
      }
    }
  }
  using R=vector<pair<pair<ll,ll>,pair<flow_t,cost_t>>>;
  R restore() {
    R ret;
    for(ll i = 0; i < graph.size(); i++) {
      for(auto &e : graph[i]) {
        if(e.isrev) continue;
        auto &rev_e = graph[e.to][e.rev];
        ret.emplace_back(make_pair(i,e.to),make_pair(rev_e.cap,e.cost));
      }
    }
    return ret;
  }
};
struct HopcroftKarp {
  vector< vector< int > > graph;
  vector< int > dist, match;
  vector< bool > used, vv;

  HopcroftKarp(int n, int m) : graph(n), match(m, -1), used(n) {}

  void add_edge(int u, int v) {
    graph[u].push_back(v);
  }

  void bfs() {
    dist.assign(graph.size(), -1);
    queue< int > que;
    for(int i = 0; i < graph.size(); i++) {
      if(!used[i]) {
        que.emplace(i);
        dist[i] = 0;
      }
    }

    while(!que.empty()) {
      int a = que.front();
      que.pop();
      for(auto &b : graph[a]) {
        int c = match[b];
        if(c >= 0 && dist[c] == -1) {
          dist[c] = dist[a] + 1;
          que.emplace(c);
        }
      }
    }
  }

  bool dfs(int a) {
    vv[a] = true;
    for(auto &b : graph[a]) {
      int c = match[b];
      if(c < 0 || (!vv[c] && dist[c] == dist[a] + 1 && dfs(c))) {
        match[b] = a;
        used[a] = true;
        return (true);
      }
    }
    return (false);
  }

  int bipartite_matching() {
    int ret = 0;
    while(true) {
      bfs();
      vv.assign(graph.size(), false);
      int flow = 0;
      for(int i = 0; i < graph.size(); i++) {
        if(!used[i] && dfs(i)) ++flow;
      }
      if(flow == 0) return (ret);
      ret += flow;
    }
  }

  void output() {
    for(int i = 0; i < match.size(); i++) {
      if(~match[i]) {
        cout << match[i] << "-" << i << endl;
      }
    }
  }
};
int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res=0,buf=0;
  bool judge = true;
  ll lim=1e7+30;
  //ll lim=1e3;cout<<"naosu"<<endl;
  auto pr=prime_table(lim);
  /*vector<ll>p;
  rep(i,0,lim)if(pr[i])p.PB(i);
  ll sz=p.size();*/
  ll n;cin>>n;
  vector<ll>x(n);
  rep(i,0,n)cin>>x[i];
  vector<vector<ll>>y(2);
  rep(i,0,n){
    if(i==0||x[i]-1>x[i-1])y[x[i]&1].PB(x[i]);
    if(i==n-1||x[i]+1<x[i+1])y[~x[i]&1].PB(x[i]+1);
  }
  pr[2]=false;
  vector<ll>sz(2);
  rep(i,0,2)sz[i]=y[i].size();
  HopcroftKarp fl(sz[0],sz[1]);
  rep(i,0,sz[0])rep(j,0,sz[1]){
    ll dif=abs(y[0][i]-y[1][j]);
    ll cost=0;
    if(pr[dif])fl.add_edge(i,j);
    //cout<<dif spa cost spa y[i] spa y[j]<<endl;
    //fl.add_edge(i,j+sz[0],1,cost);
  }
  /*ll S=sz[0]+sz[1],T=sz[0]+sz[1]+1;
  rep(i,0,sz[0]){
    fl.add_edge(S,i,1,0);
    fl.add_edge(i+sz[0],T,1,0);
  }*/
  //cout<<sz<<endl;
  //debug(y,sz);
  ll lk=fl.bipartite_matching();
  //assert(~lk&1);
  ll r1=sz[0]-lk,r2=sz[1]-lk;
  ll add=0;
  if(r1&1)add+=3;
  //cout<<lk spa r1 spa r2<<endl;
  cout<<lk+r1/2*2+r2/2*2+add<<endl;
  //map<P,ll>mp;
  /*auto r=fl.restore();
  for(auto z:r){
    if(z.se.fi==1&&z.fi.fi<sz[0]&&z.fi.se<2*sz[0]){
      cout<<y[0][z.fi.fi] spa y[1][z.fi.se-sz[0]]<<endl;
      //assert(y[z.fi.fi]!=y[z.fi.se-sz]);
      //mp[minmax(y[z.fi.fi],y[z.fi.se-sz])]++;
    }
  }*/
  //for(auto z:mp)assert(~z.se&1);
  return 0;
}

Measured memory_bytes: 4472