#include<bits/stdc++.h>
using namespace std;
int main(){
	int i,n,a=0,o=0;
	cin>>n;
	for(i=1;i<=n;i++){
		i=i+1;
		a=a+i;
	}
	a=a-50;
	cout<<a;
} 
/**************************************************************
	Problem: 1110
	User: huxuanchen
	Language: C++
	Result: Wrong Answer
****************************************************************/