/* -*- coding: utf-8 -*- * * 2937.cc: No.2937 Sigma Plus Problem - yukicoder */ #include #include using namespace std; /* constant */ /* typedef */ using ull = unsigned long long; /* global variables */ /* subroutines */ /* main */ int main() { ull n; scanf("%llu", &n); ull s = (n & 1) ? n * ((n + 1) / 2) : (n / 2) * (n + 1); printf("%llu\n", s); return 0; }