int main(){
    int counter = 0;
    while (counter < 21){
        ++counter;
        //Here we are using the bitwise and operation and is performing the and
        //operation on every bit of the counter and the second operand "1" in this case
        // 0001 0010 == 18    0001 0101 == 21
        // 0000 0001 == 1     0000 0001 == 1
        // ---------------    ---------------
        // 0000 0000 == 0     0000 0001 == 1
        if ( counter & 1) != 0){
          /*do something here when counter is odd*/
        }
        else {
            /*do something here when counter is even*/
        }
    }
    return 0;
}