How do I fix this exercise? Because the result still wrong but I don't no why

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C++

// armstrong number is a number = sum of cubes of its digit 
// ex: 153 = 1^3 + 5^3 + 3^3 --> 153 is an armstrong number
//write a program to input two natural m and n, then find an display to screen all Armstrong numbers from m to n.

#include<iostream>
#include<math.h>
using namespace std;
int main() {
	int m, n;
	cout << "Enter a natural m: ";
	cin >> m;
	cout << "Enter a natural n: ";
	cin >> n;
	
	cout << "Armstrong numbers from " << m << " to " << n << " are: " << endl;
	
	int i = m;
	while(i <= n) {
		int num = i;
		int temp = num;
		int sum = 0;
		int cnt = 0;
		while(num > 0) { // dem so luong chu so trong num
			++cnt;
			num /= 10;
		}
		num = temp;
		while(num != 0) {
			sum += pow(num%10, cnt);
			num /= 10;
		}
		if(sum == i) {
			cout << i << " ";
		}
		++i;	
	}
	return 0;
}

What I have tried:

i just want to discuss this exercise only in main function, using while (or do-while) loop (because my teacher asked me to do so .-.)
i have to watch a lot on youtube and also ask chatgpt, but it not work very well (not match the teacher's requirements)