const int  Sensorstart = 8; 
const int  Sensorstop = 9; 
const int  ledstart = 6;
const int  ledstop = 7;
   
int SensorstartState = 0;
int SensorstopState = 0;         
int lastSensorstartState = 0;  
int lastSensorstopState = 0;  
unsigned int wave;
unsigned long a,b,T;  
float t;
void setup() {
  pinMode(ledstart, OUTPUT);
  pinMode(ledstop, OUTPUT);
  pinMode(Sensorstart, INPUT);
  pinMode(Sensorstop, INPUT);
  digitalWrite(ledstart, HIGH);
  digitalWrite(ledstop, HIGH);
  Serial.begin(9600);
}

void loop() {
  
  SensorstartState = digitalRead(Sensorstart);
  if (SensorstartState != lastSensorstartState) {
    if (SensorstartState == HIGH) {
        a=millis();
        Serial.println("Gooooo.....");
        digitalWrite(ledstart, LOW);
    } 
    delay(50);
  }
  lastSensorstartState = SensorstartState;

  SensorstopState = digitalRead(Sensorstop);
  if (SensorstopState != lastSensorstopState) {
    if (SensorstopState == HIGH) {
        b=millis();
        t=(float)(b-a)/1000;
        Serial.println("stop!");
        Serial.print("t=");
        Serial.print(t);
        Serial.println("sekon");
        digitalWrite(ledstop, LOW);
    } 
    delay(50);
  }
  lastSensorstopState = SensorstopState;

}