3.cpp

#include <iostream>
#include <math.h>
#include <iomanip>

using namespace std;

int main(){
    int digits;
    cout << "Enter the number of digits (taken from 2, 4, 6, 8): ";
    cin >> digits;
    int max = pow(10,digits) - 1, c = pow(10,digits/2);
    for (int i=0; i <= max; i++) {
        int a=i%c,b=i/c;
        if (pow(a+b, 2) == i) {
            cout << setw(digits) << setfill('0') << i << "\n";
        }
    }
}