int LED = 7;
int btn = 4;
void setup () {
 // put your setup code here, to run once:
 pinMode(LED, OUTPUT);
 pinMode(btn, INPUT_PULLUP);
 digitalWrite (LED, LOW);
}

void loop() {
  // put your main code here, to run repeatedly:
if (digitalRead(btn)==LOW){
  digitalWrite(LED, HIGH);
  delay (2500);
  digitalWrite(LED, LOW);
  delay (400);
  digitalWrite(LED, HIGH);
  delay (2500);
  digitalWrite(LED, LOW);
}
}