题解 | 小乐乐求和

const rl = require('readline').createInterface({ input: process.stdin });
const iter = rl[Symbol.asyncIterator]();
const readline = async () => (await iter.next()).value;

(async () => {
    const num = BigInt(await readline());
    if (num >= 1n && num <= 10n ** 9n) {
        // 等差数列公式：n(n+1)/2
        const sum = num * (num + 1n) / 2n;
        console.log(sum.toString()); // 精确输出
    } else {
        console.log('非法数字');
    }
    rl.close();
})();