#include<iostream> #include<cstdio> //scanf()\printf() #include<cstring> #include<cmath> //sqrt()\pow()\abs()\ceil()\floor()\max()\min() using namespace std; int main(){ int n,a=1,b=0; cin>>n; for(int i=2;i<=n;i++){ a=a+b; b=b+1; } cout<<a; return 0; }