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



void main() {
    string input;

    while ((input = readln.chomp).length != 0) {
        auto n = input.to!int;
        auto sum = 0;
        auto a = 2;

        sum += a;
        if (n >= 2) {
            sum += (n+2)*(n-1)/2;
        }

        writeln(sum);
    }
}