#include <iostream>

using namespace std;

int main () {
    char ch;
    int num_to_build = 0;

    cout << "Enter a number " << endl;

    // eat the blank spaces first

    cin.get(ch);
    while (' '== ch) {
       cin.get(ch);
    }

    // we have a non-blank space.
    // read while we have a digit.

    while (('0' <= ch) && (ch <= '9') ) {
       num_to_build = num_to_build * 10 + (ch - '0');
       cin.get(ch);
    }

    cout << "You entered " << num_to_build << endl;
}