int x = 7; // This is global variables

void setup() { 
  // put your setup code here, to run once:
int x, y;  //This is local variables. 
//If the names of local & global variables are similar, then the local variables will overwrite the code of global.

pinMode(LED_BUILTIN, OUTPUT); // khởi chạy digital pin LED_BUILTIN làm OUTPUT

digitalWrite(LED_BUTTON, HIGH); // Turn the led on (HIGH is the voltage level)
delay(1000); // wait for a second
digitalWrite(LED_BUTTON, HIGH); // Turn the led off by making the voltage LOW
delay(1000); // wait for a second

void loop() { 
  // put your main code here, to run repeatedly:

}