# include<iostream> using namespace std; int main(){ //ai=a(i-1)+(i-1) //a2=a1+0,a3=a2+1,a4=a3+2 int a=1,n; cin>>n; //cout<<"1:"<<a<<endl; for(int i=1;i<=n-1;i++){ a=a+i-1; } cout<<a<<endl; return 0; }