题解 | 小乐乐求和

#include<bits/stdc++.h>
using namespace std;
//纯暴力O(n)
/*
long long n,ans=0;
cin>>n;
for(int i=1;i<=n;i++){
	ans+=i;
}
cout<<ans;
*/
//暴力的优化(二分)O(n/2)
/*
long long n,ans;
cin>>n;
int l=1,r=n;
whilt(l<=n){
	if(l!=r)
	ans+=r;
	ans+=l;
}
cout<<ans;
*/
int main(){
 	//等差数列（正解）O(1)
    long long n;
    cin>>n;
    cout<<n*(n+1)/2;
}