#include<iostream>
using namespace std;
const int ILLEGAL_INPUT = -1;
int MixNewNum(int num);
void main()
{
    int num;
    int res;
    cout << "Please enter a natural numbers: ";
    cin >> num;
    res = MixNewNum(num);
    if (res != ILLEGAL_INPUT)
        cout << "The new number is : " << res << endl;
    else
        cout << "Eror!!The input is illegal! "<< endl;
}
int MixNewNum(int num)
{
    if (num <= 0)
        return ILLEGAL_INPUT;
    int res;
    int digit1, digit2;
    int temp_num;
    int mult;
    mult = 1;
    res = 0;
    if (num > 9)
    {
        while (num > 9)
        {
            digit1 = num % 10;
            num /= 10;
            digit2 = num % 10;
            num /= 10;
            temp_num = (digit1 * 10) + (digit2);
            res = res + (temp_num*mult);
            mult *= 100;
        }

        res = res + (num*mult);

    }
    else
    {
        res = num;
    }
    return res;
}