int ledPin = 13;
int buttonPin = 5;
boolean current;
boolean previous = false;
boolean state = false;

void setup() {
  // put your setup code here, to run once:
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
}

void loop() {
  // put your main code here, to run repeatedly:
  current = digitalRead(buttonPin);
  if (!current && previous) {
    state = !state;
  }
  digitalWrite(ledPin, state);
  previous = current;
  delay(50);
}