#include <iostream>
using namespace std;
int main() {
// your code goes here
int n;
cin >> n;
while (n >= 0)
{
cout << (n*n + n)/2 + 1 << endl;
cin >> n;
}
return 0;
}
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MAoxCjIKMwo0CjUKNgo3CjgKOQoxMAoxMQoxMgoxMwoxNAoxNQoxNgoxNwoxOAoxOQoyMAoyMQoyMgoyMwoyNAoyNQoyNgoyNwoyOAoyOQozMAozMQozMgozMwozNAozNQozNgozNwozOAozOQo0MAo0MQo0Mgo0Mwo0NAo0NQo0Ngo0Nwo0OAo0OQo1MAo1MQo1Mgo1Mwo1NAo1NQo1Ngo1Nwo1OAo1OQo2MAo2MQo2Mgo2Mwo2NAo2NQo2Ngo2Nwo2OAo2OQo3MAo3MQo3Mgo3Mwo3NAo3NQo3Ngo3Nwo3OAo3OQo4MAo4MQo4Mgo4Mwo4NAo4NQo4Ngo4Nwo4OAo4OQo5MAo5MQo5Mgo5Mwo5NAo5NQo5Ngo5Nwo5OAo5OQotNjY2Cg==
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-666