string add(string a, string b) {
	reverse(a)
	reverse(b)
	n = a.size
	m = b.size
	string ans(max(n, m))

	bool carry = false;
	for i = 0; i < min(n, m); i += 1 {
		ans[i] = (a[i] + b[i] + carry) % 10
		carry = (a[i] + b[i]) >= 10
	}
	for i = min(n, m); i < n; i += 1 {
		ans[i] = a[i] + carry
		carry = false
	}
	for i = min(n, m); i < m; i += 1 {
		ans[i] = b[i] + carry
		carry = false
	}
	if (carry)
		ans = ans + '1'
	reverse(ans)
	return ans
}