Button OnClick function?

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using System;
 
 public class Codice : MonoBehaviour {
  public Text Inputfield;
  public Text Inputfield1;
  public Button cmdCalcola;
  
  public void Start()
  {
      var btt = gameObject.GetComponent<Button>();
      int n = System.Int32.Parse(Inputfield.text.ToString());
      int n1 = System.Int32.Parse(Inputfield1.text.ToString());
      string ris = ((n+n1)/2).ToString();
      btt.onClick.AddListener(() => {
          function(ris);
      });    
  }   
  void function(string a)
  {
      Debug.Log("The medium is: "+a);
  }

}

Why it gives t$$anonymous$$s error in Console:" FormatException: Input string was not in the correct format System.Int32.Parse (System.String s) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/Int32.cs:629) Codice.Start () (at Assets/Codici/Codice.cs:16) ". How could i start t$$anonymous$$s function. Excuse for my English