text: N, K, M, R = map(int, input().split())
  
  S = []
  for _ in range(N-1):
      S.append(int(input()))
  
  S.sort(reverse=True)
  
  if sum(S[:K])/K >= R:
      print(0)
  else:
      if R*K-sum(S[:K-1]) > M:
          print(-1)
      else:
          print(R*K-sum(S[:K-1]))
  