#define INPUT_DIGIT_LIMIT 5

void setup(){
  Serial.begin(9600);
}

void loop(){
  Serial.print("Send any number 0-");
  Serial.print(INPUT_DIGIT_LIMIT);
  Serial.print(" digits. ");
  int num=getInput();
  Serial.print("Received number: ");
  Serial.println(num);
}

int getInput(){
   int counter=0;
   char inputByte;
   char inputBytes[INPUT_DIGIT_LIMIT+1];
   do{
     while(!Serial.available());
       inputByte=Serial.read();
       inputBytes[counter++]=inputByte;
       if(inputByte==10||inputByte==13||counter==INPUT_DIGIT_LIMIT)
         break;
   }while(true);
   inputBytes[counter]='\n';

   return atoi(inputBytes);
}