蒟蒻求助，最后三个点TLE

最后三个点TLE，请问怎么优化？？？

#include <iostream>
#include <cmath>
#include <string>
using namespace std;
bool isPrime(int n) {
    if (n <= 1)
        return false;
    for (int i = 2; i <= sqrt(n); i++) {
        if (n % i == 0)
            return false;
    }
    return true;
}
bool isPalindrome(int d) {
    string s = to_string(d);
    int left = 0, right = s.size() - 1;
    while (left < right) if (s[left++] != s[right--]) return false;
    return true;
}
int main() {
    int a, b;
    cin >> a >> b;
    for (; a <= b; a++) if (isPrime(a) && isPalindrome(a)) cout << a << endl;
    return 0;
}