Code mẫu

#include <bits/stdc++.h>

using namespace std;

bool prime(int x)
{
    if (x < 2) return 0;
    for (int i = 2; i * i <= x; ++i)
        if (x % i == 0) return 0;
    return 1;
}

int main()
{
    freopen("RESORT.INP", "r", stdin);
    freopen("RESORT.OUT", "w", stdout);
    int n, one = 0, two = 0;
    cin >> n;
    for (int i = 1; i <= n; ++i)
    {
        int x;
        cin >> x;
        one += (x == 1);
        two += (x == 2);
    }

    vector <int> a(n + 10);
    int l = 0;
    if (two) a[++l] = 2, --two;
    if (one) a[++l] = 1, --one;
    for (int i = 1; i <= two; ++i) a[++l] = 2;
    for (int i = 1; i <= one; ++i) a[++l] = 1;
    int ans = 0;
    for (int i = 1; i <= n; ++i)
        ans += (prime(a[i] += a[i - 1]));
    cout << ans;
    return 0;
}