#include <iostream>
using namespace std;
int main()
{
int n;
while (cin>>n)
{
if (n%2) //陷阱在于n*(n+1)会导致int溢出
{
cout<<((n+1)/2)*n<<endl<<endl;
}
else
{
cout<<(n/2)*(n+1)<<endl<<endl;
}
}
return 0;
}
原文：http://www.cnblogs.com/AC-LG/p/4645511.html