Find the law - dividing plane

n straight lines up plane can be divided into several parts?

Input formats:

Plural sets of inputs, each input a positive integer n (n≤1000000), n straight lines expressed.

Output formats:

Each input and output a positive integer, each output per line, up to the plane into several parts n straight lines.

Sample input:

1 2

Sample output:

2 4

1: 2
2: There is a point of intersection with the first, separated two new 2 + 2 = 4
3: Up to now the two intersections of two lines, the newly added three surfaces 2 + 3 + 2 = 7
4: with 3 intersections three lines, the newly added four faces 2 + 3 + 4 + 2 = 11

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<string>
#include<cmath>
#include<queue>
#include<map>
#include<set>
#define ll long long
#define pr make_pair
#define maxn 100005
using namespace std;

int main(void){
    int n;
    while(cin >> n){
        ll ans = (1+n)/2.0*n+1;
        cout << ans << endl;
    }
    return 0;
}

Note: /2.0