#include <bits/stdc++.h>

using namespace std;

#define int long long
#define FOR(i, a, b) for(ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for(ll i = (a); i > (b); --i)
#define REP(i, n) for(ll i = 0; i < (n); ++i)
#define REPR(i, n) for(ll i = n; i >= 0; i--)
#define FOREACH(x, a) for(auto &(x) : (a))
#define VECCIN(x)                                                              \
    for(auto &youso_ : (x)) cin >> youso_
#define bitcnt __builtin_popcount
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
template <typename T = long long> inline T IN() {
    T x;
    cin >> x;
    return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&... tail) {
    cin >> head;
    CIN(move(tail)...);
}
#define CINT(...)                                                              \
    int __VA_ARGS__;                                                           \
    CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
    double __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
    ll __VA_ARGS__;                                                            \
    CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
    string __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
    {                                                                          \
        FOREACH(x, v) { cout << x << " "; }                                    \
        cout << "\n";                                                          \
    }
template <typename T = string> inline void eputs(T s) {
    cout << s << "\n";
    exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
    std::fill((T *)array, (T *)(array + N), val);
}
template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef unsigned long long ul;
typedef vector<ll> VL;
typedef pair<ll, ll> PL;

const int INF = 1e9;
const int MOD = 1e9 + 7;
const double PI = atan(1.0) * 4.0;
// const int MOD = 998244353;
const ll LINF = 9e18;
const ll dx[] = {1, 0, -1, 0};
const ll dy[] = {0, 1, 0, -1};

void cinfast() {
    cin.tie(0);
    ios::sync_with_stdio(false);
}

struct edge {
    ll to, cap, rev;
};

struct Mflow {
    vector<vector<edge>> G;
    VL level, iter;
    ll N;

    Mflow(ll size) : N(size) { init(size); };

    void init(ll size) { G.resize(N); }

    void add_edge(ll from, ll to, ll cap) {
        G[from].push_back({to, cap, (ll)G[to].size()});
        G[to].push_back({from, 0, (ll)G[from].size() - 1});
    }
    void bfs(ll s) {
        level.assign(N, -1);
        queue<ll> q;
        level[s] = 0;
        q.push(s);
        while(!q.empty()) {
            ll now = q.front();
            q.pop();
            FOREACH(e, G[now]) {
                if(e.cap > 0 && level[e.to] == -1) {
                    level[e.to] = level[now] + 1;
                    q.push(e.to);
                }
            }
        }
    }

    ll dfs(ll now, ll t, ll f) {
        if(now == t) return f;
        for(ll &i = iter[now]; i < G[now].size(); i++) {
            edge &e = G[now][i];
            if(e.cap > 0 && level[now] < level[e.to]) {
                ll d = dfs(e.to, t, min(f, e.cap));
                if(d > 0) {
                    e.cap -= d;
                    G[e.to][e.rev].cap += d;
                    return d;
                }
            }
        }
        return 0;
    }

    ll max_flow(ll s, ll t) {
        ll flow = 0;
        while(1) {
            bfs(s);
            if(level[t] < 0) return flow;
            iter.assign(N, 0);
            ll f;
            while((f = dfs(s, t, LINF)) > 0) {
                flow += f;
            }
        }
    }
};

ll m, n;
map<ll, ll> com;

bool is_prime[(ll)1e7 + 1];
bool is_prime_small[(ll)1e7 + 1];

VL seg_sieve(ll a, ll b) {
    VL res;
    for(ll i = 2; i * i < b; i++) is_prime_small[i] = true;
    REP(i, b - a) is_prime[i] = true;
    for(ll i = 2; i * i < b; i++) {
        if(is_prime_small[i]) {
            for(ll j = 2 * i; j * j < b; j += i) is_prime_small[j] = false;
            for(ll j = max(2LL, (a + i - 1) / i) * i; j < b; j += i)
                is_prime[j - a] = false;
        }
    }
    REP(i, b - a) if(is_prime[i]) res.emplace_back(i + a);
    return res;
}

signed main() {
    LCIN(N);
    REP(i, N) {
        LCIN(x);
        com[x] ^= 1;
        com[x + 1] ^= 1;
    }
    VL odd, even;
    FOREACH(x, com) {
        if(x.second) {
            if(x.first % 2)
                odd.emplace_back(x.first);
            else
                even.emplace_back(x.first);
        }
    }
    ll o = odd.size(), e = even.size();
    Mflow mf(o + e + 2);
    REP(i, o) mf.add_edge(o + e, i, 1);
    REP(i, e) mf.add_edge(o + i, o + e + 1, 1);
    seg_sieve(0, 10000001);
    REP(i, o) REP(j, e) {
        if(is_prime[abs(odd[i] - even[j])] && abs(odd[i] - even[j]) > 2) {
            mf.add_edge(i, o + j, 1);
        }
    }
    ll ans = mf.max_flow(o + e, o + e + 1);
    cout << ans + (o - ans) / 2 * 2 + (e - ans) / 2 * 2 + (o - ans) % 2 * 3
         << "\n";
}


Measured memory_bytes: 22000