const fs = require('fs');
function calcCoins(k) {
let totalCoins = 0;
let [day, coinOfDay] = [1, 1];
while (day <= k) {
let p = coinOfDay;
while (p > 0) {
totalCoins += coinOfDay;
[p, day] = [p - 1, day + 1];
if (day > k) break;
}
coinOfDay++;
}
return totalCoins;
}
console.log(calcCoins(parseInt(fs.readFileSync(0, 'utf-8'))));