float InputTXT_Float(){
float val;
    while (Serial.available() == 0) ;
  {
    val = Serial.parseFloat();
    while (Serial.available() > 0)
      { Serial.read() ; }
  } 
  return val;
}

void setup() {
    Serial.begin(9600);
      float sum,N,i ;
    Serial.print("Enter a score,or -1 to quit : ");
      N = InputTXT_Float();
    Serial.println(N);
      sum=0;
      while(N>0) {
      sum=sum+N;
    Serial.print("Enter a score,or -1 to quit : ");
      N = InputTXT_Float();
    Serial.println(N);
      i++; }
    Serial.print("Number of students is ");
    Serial.println(i,0);
    Serial.print("Average is ");
    Serial.println(sum/i);
}
void loop() {}