text: # -*- coding: utf-8 -*-
  
  import math
  import collections
  import bisect
  import heapq
  import time
  import random
  
  """
  created by shhuan at 2017/10/4 19:43
  
  """
  
  a, b, f, k = map(int, input().split())
  
  if b < f or f+b < a:
      print(-1)
      exit(0)
  
  r = a-f
  
  res = 0
  fuel = b
  for i in range(k):
      if i % 2 == 0:
          # 0->a
          if i == k-1:
              if fuel >= a:
                  break
              elif fuel >= f:
                  res += 1
                  break
              else:
                  print(-1)
                  exit(0)
          else:
              if fuel < f:
                  print(-1)
                  exit(0)
              if fuel >= a + r:
                  fuel -= a
              else:
                  res += 1
                  fuel = b - r
      else:
          # a->0
          if i == k-1:
              if fuel < r:
                  print(-1)
                  exit(0)
              if fuel >= a:
                  break
              elif fuel >= r:
                  res += 1
                  break
              else:
                  print(-1)
                  exit(0)
          else:
              if fuel >= a + f:
                  fuel -= a
              else:
                  res += 1
                  fuel = b - f
  print(res)