#include <iostream>

int main() {
    int n;
    std::cin >> n;

    if (n < 1 || n > 1000000) {
        goto invalidInput;
    }

    int count = 0;
    int i = 2;

startLoop:
    if (i <= n) {
        count++;
        i += 2;
        goto startLoop;
    }

    std::cout << count << std::endl;
    return 0;

invalidInput:
    std::cerr << "Invalid input" << std::endl;
    return 1;
}