https://www.acmicpc.net/problem/1789
let fs = require('fs');
let input = fs.readFileSync('/dev/stdin').toString();
let n = Number(input);
function answer(num) {
let sum = 0;
let idx = 1;
while (sum < num) {
sum += idx++;
}
if (sum > num) return idx-2;
return idx-1;
}
console.log(answer(n));