const int btnPin=15;
int num=0;
bool flag=false;
void echo(){
  flag=true;
  num++;
}
void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(btnPin, INPUT);
  attachInterrupt(btnPin, echo, CHANGE);
}

void loop() {
  // put your main code here, to run repeatedly:
  if (flag) {
  Serial.println(num);
  num=0;
  }
  flag=false;
}