40ptsMLE求Debug

#include<iostream>
#include<string>
#include<stack>
using namespace std;
string backward(string s) {
	stack<char> letters;
	string ans = "";
	for (int i = 0; i < (int)s.size(); i++) letters.push(s[i]);
	for (int i = 0; i < (int)s.size(); i++) {
		ans += letters.top();
		letters.pop();
	}
	return ans;
}
string F(string s) {
	char last;
	string other = "";
	for (int i = 0; i < (int)s.size() - 1; i++) other += s[i];
	last = s[(int)s.size() - 1];
	return s + last + other;
}
int main() {
	int n;
	string s;
	cin >> s >> n;
	for (; (int)s.size() < n; s = F(s));
	cout << s[n - 1];
	return 0;
}