// Arup Guha
// 6/13/2024
// Solution to Kattis Problem: Longest Prime Sum
// https://open.kattis.com/problems/longestprimesum

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

int main() {

    // Answer is all 2s and maybe 1 3...
    ll n;
    cin >> n;
    cout << n/2 << endl;
    return 0;
}