#include <iostream>

using namespace std;

void ex0() {
    cout << (char)43;
}

void ex1() {
    int a;
    cin >> a;
    cout << a * (1 + a) / 2; // sum of arithmetic sequence, where a0 = 1 and d = 1
}

void ex2() {
    int a;
    cin >> a;
    cout << ((a % 3 == 0) != (a % 5 == 0)); // != is logical XOR
}

void ex3() {
    int a;
    cin >> a;
    cout << a / 3 + a / 5 - (a / 5) / 3;
}

void ex4() {
    int a;
    cin >> a;
    a /= 3;
    cout << (6 + (a - 1) * 3) * a / 2; // sum of arithmetic sequence, where a0 = 3 and d = 3
}

// Building and running in PowerShell:
// g++ .\*.cpp -o out.exe; if($?) { .\out.exe }
int main() {
    ex4();
}