<?xml version="1.0" encoding="utf-8"?>
  <android.support.constraint.ConstraintLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      tools:context=".MainActivity">
  
      <Button
          android:id="@+id/start"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="start campaign"
          app:layout_constraintBottom_toBottomOf="parent"
          app:layout_constraintLeft_toLeftOf="parent"
          app:layout_constraintRight_toRightOf="parent"
          app:layout_constraintTop_toTopOf="parent"/>
  
  </android.support.constraint.ConstraintLayout>