function Main (input) {
    var n = parseInt(input) ;
    var s = 0;
    if ( n % 2 == 0) {
        s = (1 + n) * n / 2
        console.log(s)
} else {
    s = (1 + n) * (n - 1) / 2 + (1 + n) / 2
    console.log(s)
}
}