title: How to add to number in label in kivy?
body: <p>I am trying to update my label in this way, I have a label and a function, when I use this function, my label add a number in it's text. In this way, if my label is 1 before I click the button, after I click a button, the label change to 1+x. I have no idea how can I do this. It's pure algebra.</p>
  
  <p>.py</p>
  
  <pre><code>class PrimeiroScreen(Screen):
  def __init__(self,**kwargs):
      self.name = 'uno'
      super(Screen,self).__init__(**kwargs)
  
  def fc(self):
      self.ids.lb1.text += "1" #its add 1 in the label, but not sum 1 to label value
  </code></pre>
  
  <p>and.kv</p>
  
  <pre><code>&lt;PrimeiroScreen&gt;:
  GridLayout:
      cols: 1     
      size_hint: (.3, .1)
      pos_hint:{'x': .045, 'y': .89}
      Label:
          text:"0"
          font_size: '30dp'
          text_size: self.width, self.height
          id: lb1
      Button:
          text: "Somar 3"
          font_size: '30dp'
          text_size: self.width - 50, self.height
          on_press: root.fc()
  </code></pre>