求助！！！TLE四个点。。。Why？

#include <bits/stdc++.h>
using namespace std;

bool prime(int a) {
	bool r = true;
	for (int i = 2; i <= a - 1; i++) {
		if (a % i == 0) {
			r = false;
			break;
		}
	}
	return r;
}

int main() {
	int a, b, number, nnn;
	string n, nn;
	cin >> a >> b;
	for (int i = a; i <= b; i++) {
		number = i;
		nn = to_string(number);
		n = "";
		for (int j = to_string(number).length() - 1; j >= 0; j--) {
			n += to_string(number)[j];
		}
		nnn = stoi(n);
		if ((nnn == i) && prime(i)) {
			cout << i << '\n';
		}
	}
	return 0;
}