#include <bits/stdc++.h>

using namespace std;

bool CheckSNT(int n)

{

    if (n < 2) return false;

    for (int i = 2; i <= sqrt(n); i++)

        if (n % i == 0) return false;

    return true;

}

$\\$

int main()

{

    int n; cin >> n;

    n --;

    while (!CheckSNT(n))

    {

        n --;

    }

    cout << n;

}

#include <bits/stdc++.h>
#define int long long
using namespace std;
vector<bool> Check(int n)
{
    vector<bool> a(n + 1, true);
    a[0] = a[1] = false;
    for (int i = 2; i * i <= n; i++)
    {
        if (a[i])
        {
            for (int j = i * i; j <= n; j += i) a[j] = false;
        }
    }
    return a;
}
signed main()
{
    int n; cin >> n;
    if (n < 2)
    {
        cout << -1;
        return 0;
    }
    vector<bool> a = Check(n);
    n--;
    while (n >= 2 && !a[n]) n--;
    if (n >= 2) cout << n;
    else cout <<-1;
}

Chúc bn học tốt!

$\color{Turquoise}{\triangleright \triangleright\ \color{ Turquoise}{\text{ThienPhuc123} \triangleleft \triangleleft}}$