process.stdin.resume();
process.stdin.setEncoding('utf8');

var lines = [];
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});
reader.on('line', (line) => {
  lines.push(line);
});
reader.on('close', () => {
    
  let sum = 0; //合計
let n = lines[0];
for (i = 0; i <= n; i++) {
    sum += i;
}

console.log(sum); //15
});