求助！！！TLE四个点。。。Why？

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;
}