#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 bytesAvailable;
   int counter=0;
   char inputByte;
   char inputBytes[INPUT_DIGIT_LIMIT+1];

   for(int i=0;i<sizeof(inputBytes);i++)
     inputBytes[i]='\n';

   do{
     while(!(bytesAvailable=Serial.available()));
     inputByte=Serial.read();
     if(counter==INPUT_DIGIT_LIMIT||inputByte==10||inputByte==13){
       while(Serial.available()){Serial.read();};
       break;
     }
     inputBytes[counter]=inputByte;
     counter++;
   }while(true);
   return atoi(inputBytes);
}