import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  TextEditingController hassasiyet = TextEditingController();
  bool islem = true;
  double pay = 4;
  double sayac = 0;


  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(

        body: Center(
          child: Column(
            children: [
              TextField(
                controller: hassasiyet,
                keyboardType: TextInputType.number,
                decoration: InputDecoration(
                  labelText: "Hassasiyet Giriniz:"
                ),
              ),
              SizedBox(height: 10),
              ElevatedButton(onPressed: () {
                setState(() {
                  sayac = 0;
                  islem = true;
                  int hass = int.tryParse(hassasiyet.text) ?? 0;

                  for(var i = 0; i < hass; i++){
                    double payda = 1 + (i * 2);
                    if(islem == true){
                      sayac = sayac + (pay / payda);
                      islem = false;
                    }
                    else{
                      sayac = sayac - (pay / payda);
                      islem = true;
                    }

                  }
                });
              }, child: Text("Hesapla"),),
              

              Text("$sayac")

            ],
          ),
        ),
      ),
    );
  }
}