import std.stdio,std.string,std.conv;

int main()
{
	int a[10001];
	a[0] = 0;
	a[1] = 2;
	foreach(i;2..10001)
	{
		a[i] = a[i-1] + i;
	}

	string s;
	while((s = readln.chomp).length != 0)
	{
		int n = s.to!int;
		a[n].writeln;
	}
	return 0;
}