#include <iostream>
using namespace std;

int main(void) {
    int x;  cin >> x;
    
    if (x == 2) {
        cout << 2;
    }
    else if (x % 2 == 0) {
        cout << (4 + 10 * (x / 2 - 2));
    }
    else {
        cout << (5 + 10 * ((x - 1) / 2 - 1));
    }
    cout << endl;

    return 0;
}