#include<iostream>

int main(){
    unsigned long long n; std::cin >> n;
    unsigned long long ans = n * (n + 1) / 2;
    unsigned long long dummy;
    if (__builtin_mul_overflow(n, n + 1, &dummy)){
        ans += (1ULL << 63);
    }
    std::cout << ans << std::endl;
}