华为oj之等差数列前n项和

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#include<cstdio>
#include<iostream>
using namespace std;

int main()
{
    int n;
    while(cin>>n)
    {
        if(n <= 0) return (-1);
        int sum;
        sum = n*(3*n+1)/2;

        cout<<sum<<endl;
    }    
    return 0;
}