const unsigned int BUTTON_PIN = 7;
const unsigned int LED_PIN = 13;

int oldButtonState = LOW;
int ledState = LOW;

void setup() {
    pinMode(LED_PIN, OUTPUT);
    pinMode(BUTTON_PIN, INPUT);
}

void loop() {
    const int CURRENT_BUTTON_STATE = digitalRead(BUTTON_PIN);

    if (CURRENT_BUTTON_STATE != oldButtonState &&
        CURRENT_BUTTON_STATE == HIGH) {
        if (ledState == LOW) {
            ledState = HIGH;
        } else {
            ledState = LOW;
        }

        digitalWrite(LED_PIN, ledState);
        delay(50);
    }

    oldButtonState = CURRENT_BUTTON_STATE;
}