title: Arduino - Counter:50: error: lvalue required as left operand of assignment
body: <p>This is my code it is for my Sainsmart Arduino Uno.:</p>
  
  <pre><code>void buttonChange() {
    if (digitalRead(button) == LOW) {
      ltp2 = ltp;
      ltp = millis();
      if (number == 0 || ltp -= ltp2 &gt; 500){ 
        number += 1;
        displayNumber();
      }
    }
  }
  </code></pre>
  
  <p>I receive this error:</p>
  
  <pre><code>counter.ino: In function 'void buttonChange()':
  counter:50: error: lvalue required as left operand of assignment
  </code></pre>
  
  <p>Thanks for the help.</p>