#pragma region Macros
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define rep2(i, a, b) for(ll i = a; i <= b; ++i)
#define rep(i, n) for(ll i = 0; i < n; ++i)
#define rep3(i, a, b) for(ll i = a; i >= b; --i)
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vll vector<ll>
#define vpi vector<pii>
#define vpll vector<pll>
#define overload2(_1, _2, name, ...) name
#define vec(type, name, ...) vector<type> name(__VA_ARGS__)
#define VEC(type, name, size)                                                                                                                                  \
    vector<type> name(size);                                                                                                                                   \
    IN(name)
#define vv(type, name, h, ...) vector<vector<type>> name(h, vector<type>(__VA_ARGS__))
#define VV(type, name, h, w)                                                                                                                                   \
    vector<vector<type>> name(h, vector<type>(w));                                                                                                             \
    IN(name)
#define vvv(type, name, h, w, ...) vector<vector<vector<type>>> name(h, vector<vector<type>>(w, vector<type>(__VA_ARGS__)))
#define fi first
#define se second
#define all(c) begin(c), end(c)
#define ios ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
#define lb(c, x) distance((c).begin(), lower_bound(all(c), (x)))
#define ub(c, x) distance((c).begin(), upper_bound(all(c), (x)))
using namespace std;
template <class T> using pq = priority_queue<T>;
template <class T> using pqg = priority_queue<T, vector<T>, greater<T>>;
#define Size(c) (int)(c).size()
#define INT(...)                                                                                                                                               \
    int __VA_ARGS__;                                                                                                                                           \
    IN(__VA_ARGS__)
#define LL(...)                                                                                                                                                \
    ll __VA_ARGS__;                                                                                                                                            \
    IN(__VA_ARGS__)
#define ULL(...)                                                                                                                                               \
    ull __VA_ARGS__;                                                                                                                                           \
    IN(__VA_ARGS__)
#define STR(...)                                                                                                                                               \
    string __VA_ARGS__;                                                                                                                                        \
    IN(__VA_ARGS__)
#define CHR(...)                                                                                                                                               \
    char __VA_ARGS__;                                                                                                                                          \
    IN(__VA_ARGS__)
#define DBL(...)                                                                                                                                               \
    double __VA_ARGS__;                                                                                                                                        \
    IN(__VA_ARGS__)
#define LD(...)                                                                                                                                                \
    ld __VA_ARGS__;                                                                                                                                            \
    IN(__VA_ARGS__)
int scan() { return getchar(); }
template <class T> void scan(T a) { cin >> a; }
void scan(int &a) { cin >> a; }
void scan(long long &a) { cin >> a; }
void scan(char &a) { cin >> a; }
void scan(double &a) { cin >> a; }
void scan(long double &a) { cin >> a; }
void scan(char a[]) { scanf("%s", a); }
void scan(string &a) { cin >> a; }
template <class T> void scan(vector<T> &);
template <class T, size_t size> void scan(array<T, size> &);
template <class T, class L> void scan(pair<T, L> &);
template <class T, size_t size> void scan(T (&)[size]);
template <class T> void scan(vector<T> &a) {
    for(auto &i : a) scan(i);
}
template <class T> void scan(deque<T> &a) {
    for(auto &i : a) scan(i);
}
template <class T, size_t size> void scan(array<T, size> &a) {
    for(auto &i : a) scan(i);
}
template <class T, class L> void scan(pair<T, L> &p) {
    scan(p.first);
    scan(p.second);
}
template <class T, size_t size> void scan(T (&a)[size]) {
    for(auto &i : a) scan(i);
}
template <class T> void scan(T &a) { cin >> a; }
void IN() {}
template <class Head, class... Tail> void IN(Head &head, Tail &... tail) {
    scan(head);
    IN(tail...);
}
string stin() {
    string s;
    cin >> s;
    return s;
}
template <class T> inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T> inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return 1;
    }
    return 0;
}
vi iota(int n) {
    vi a(n);
    iota(all(a), 0);
    return a;
}
template <class T> void UNIQUE(vector<T> &x) {
    sort(all(x));
    x.erase(unique(all(x)), x.end());
}
int in() {
    int x;
    cin >> x;
    return x;
}
ll lin() {
    unsigned long long x;
    cin >> x;
    return x;
}
void print() { putchar(' '); }
void print(bool a) { cout << a; }
void print(int a) { cout << a; }
void print(long long a) { cout << a; }
void print(char a) { cout << a; }
void print(string &a) { cout << a; }
void print(double a) { cout << a; }
template <class T> void print(const vector<T> &);
template <class T, size_t size> void print(const array<T, size> &);
template <class T, class L> void print(const pair<T, L> &p);
template <class T, size_t size> void print(const T (&)[size]);
template <class T> void print(const vector<T> &a) {
    if(a.empty()) return;
    print(a[0]);
    for(auto i = a.begin(); ++i != a.end();) {
        cout << " ";
        print(*i);
    }
    cout << endl;
}
template <class T> void print(const deque<T> &a) {
    if(a.empty()) return;
    print(a[0]);
    for(auto i = a.begin(); ++i != a.end();) {
        cout << " ";
        print(*i);
    }
}
template <class T, size_t size> void print(const array<T, size> &a) {
    print(a[0]);
    for(auto i = a.begin(); ++i != a.end();) {
        cout << " ";
        print(*i);
    }
}
template <class T, class L> void print(const pair<T, L> &p) {
    cout << '(';
    print(p.first);
    cout << ",";
    print(p.second);
    cout << ')';
}
template <class T> void print(set<T> &x) {
    for(auto e : x) print(e), cout << " ";
    cout << endl;
}
template <class T> void print(multiset<T> &x) {
    for(auto e : x) print(e), cout << " ";
    cout << endl;
}
template <class T, size_t size> void print(const T (&a)[size]) {
    print(a[0]);
    for(auto i = a; ++i != end(a);) {
        cout << " ";
        print(*i);
    }
}
template <class T> void print(const T &a) { cout << a; }
int out() {
    putchar('\n');
    return 0;
}
template <class T> int out(const T &t) {
    print(t);
    putchar('\n');
    return 0;
}
template <class Head, class... Tail> int out(const Head &head, const Tail &... tail) {
    print(head);
    putchar(' ');
    out(tail...);
    return 0;
}
ll gcd(ll a, ll b) {
    while(b) {
        ll c = b;
        b = a % b;
        a = c;
    }
    return a;
}
ll lcm(ll a, ll b) {
    if(!a || !b) return 0;
    return a * b / gcd(a, b);
}
vector<pll> factor(ll x) {
    vector<pll> ans;
    for(ll i = 2; i * i <= x; i++)
        if(x % i == 0) {
            ans.push_back({i, 1});
            while((x /= i) % i == 0) ans.back().second++;
        }
    if(x != 1) ans.push_back({x, 1});
    return ans;
}
vector<int> divisor(int x) {
    vector<int> ans;
    for(int i = 1; i * i <= x; i++)
        if(x % i == 0) {
            ans.pb(i);
            if(i * i != x) ans.pb(x / i);
        }
    return ans;
}
template <typename T> void zip(vector<T> &x) {
    vector<T> y = x;
    sort(all(y));
    for(int i = 0; i < x.size(); ++i) { x[i] = lb(y, x[i]); }
}
int popcount(ll x) { return __builtin_popcountll(x); }
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int n) { return uniform_int_distribution<int>(0, n - 1)(rng); }
template <typename T> void shuffle(vector<T> &v) {
    rep3(i, v.size() - 1, 1) { swap(v[i], v[rnd(i)]); }
}
#define endl '\n'
vector<string> YES{"NO", "YES"};
vector<string> Yes{"No", "Yes"};
vector<string> yes{"no", "yes"};

#ifdef _LOCAL
#undef endl
#define debug(x)                                                                                                                                               \
    cout << #x << ": ";                                                                                                                                        \
    print(x);                                                                                                                                                  \
    cout << endl;
void err() {}
template <class T> void err(const T &t) {
    print(t);
    cout << " ";
}
template <class Head, class... Tail> void err(const Head &head, const Tail &... tail) {
    print(head);
    putchar(' ');
    out(tail...);
}
#else
#define debug(x)
template <class... T> void err(const T &...) {}
#endif

template <typename T> struct edge {
    int from, to;
    T cost;
    int id;
    edge(int to, T cost) : from(-1), to(to), cost(cost) {}
    edge(int from, int to, T cost) : from(from), to(to), cost(cost) {}
    // edge(int from,int to,T cost,int id):from(from),to(to),cost(cost),id(id){}
    edge &operator=(const int &x) {
        to = x;
        return *this;
    }
    operator int() const { return to; }
};

template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedTree = vector<Edges<T>>;
template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
template <typename T> using Matrix = vector<vector<T>>;
using Graph = vector<vector<int>>;
using tree = vector<vector<int>>;

tree getTree(int n, int offset = 1) {
    tree res(n);
    for(int i = 0; i < n - 1; i++) {
        int a, b;
        cin >> a >> b;
        a -= offset, b -= offset;
        res[a].emplace_back(b);
        res[b].emplace_back(a);
    }
    return res;
}
template <typename T> WeightedTree<T> getWeightedTree(int n, int offset = 1) {
    WeightedTree<T> res(n);
    for(int i = 0; i < n - 1; i++) {
        int a, b;
        cin >> a >> b;
        a -= offset, b -= offset;
        T c;
        cin >> c;
        res[a].emplace_back(b, c);
        res[b].emplace_back(a, c);
    }
    return res;
}

Graph getGraph(int n, int m, int offset = 1) {
    Graph res(n);
    for(int i = 0; i < m; ++i) {
        int a, b;
        cin >> a >> b;
        a -= offset, b -= offset;
        res[a].emplace_back(b);
        res[b].emplace_back(a);
    }
    return res;
}

template <typename T> WeightedGraph<T> getWeightedGraph(int n, int m, int offset = 1) {
    WeightedGraph<T> res(n);
    for(int i = 0; i < m; ++i) {
        int a, b;
        cin >> a >> b;
        T c;
        cin >> c;
        res[a].emplace_back(b, c);
        res[b].emplace_back(a, c);
    }
}
struct Setup_io {
    Setup_io() {
        ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
        cout << fixed << setprecision(15);
    }
} setup_io;
#pragma endregion

// _________コーディングはここから！！___________

namespace Factorize {
using i128 = __int128_t;
ll get_factor(ll n) {
    if(n == 3 or n == 5) return 0;
    ll x, y;
    ll iter = 0;
    while(true) {
        ++iter;
        x = iter % n;
        y = (i128(iter) * i128(iter) + 1) % i128(n);
        while(1) {
            ll g = gcd(n, abs(x - y));
            if(g == n) break;
            if(g > 1) return g;
            x = (i128(x) * i128(x) + 1) % i128(n);
            y = (i128(y) * i128(y) + 1) % i128(n);
            y = (i128(y) * i128(y) + 1) % i128(n);
        }
    }
}
ll modpow(ll a, ll n, ll mod) {
    ll res = 1;
    while(n) {
        if(n & 1) res = i128(a) * i128(res) % i128(mod);
        n >>= 1, a = i128(a) * i128(a) % i128(mod);
    }
    return res;
}
bool miller_rabin_test(ll p, ll s, ll d, ll n) {
    ll x = modpow(p, d, n);
    if(x == 1) return true;
    ll y = n - 1;
    rep(i, s) {
        if(x == y) return true;
        x = i128(x) * i128(x) % i128(n);
    }
    return false;
}
bool miller_rabin(ll n) {
    if(n == 1) return false;
    ll d = n - 1, s = 0;
    while(~d & 1) d >>= 1, ++s;
    if(n < 4759123141LL) {
        for(auto p : {2, 7, 61}) {
            if(p >= n) break;
            if(!miller_rabin_test(p, s, d, n)) return false;
        }
    } else {
        for(auto p : {2, 325, 9375, 28178, 450775, 9780504, 1795265022}) {
            if(p >= n) break;
            if(!miller_rabin_test(p, s, d, n)) return false;
        }
    }
    return true;
}

map<ll, int> factorize(ll n) {
    map<ll, int> res;
    while(~n & 1) res[2]++, n >>= 1;
    queue<ll> que;
    que.emplace(n);
    while(!que.empty()) {
        ll now = que.front();
        que.pop();
        if(now == 1) continue;
        if(miller_rabin(now)) {
            res[now]++;
            continue;
        }
        ll g = get_factor(now);
        que.emplace(g);
        que.emplace(now / g);
    }
    return res;
}

bool isPrime(ll n) { return miller_rabin(n); }
} // namespace Factorize
using namespace Factorize;
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; }
        }
    }
};

main() {
    INT(n);
    VEC(int, a, n);
    const int N = 1e7 + 2;
    vi p(N);
    rep(i, n) p[a[i]]++;
    rep(i, N - 1) { p[i] = p[i + 1] ^ p[i]; }
    vi x[2];
    rep(i, N) {
        if(p[i]) x[i & 1].eb(i);
    }
    HopcroftKarp g(x[0].size(), x[1].size());
    rep(i, x[0].size()) {
        rep(j, x[1].size()) {
            int d = abs(x[1][j] - x[0][i]);
            if(isPrime(d)) g.add_edge(i, j);
        }
    }
    int d = g.bipartite_matching();
    int ans = x[0].size() + x[1].size() - d;
    if((x[0].size() - d) & 1) ans++;
    cout << ans << endl;
}


Measured memory_bytes: 42368