N = gets.to_i
A = gets.split.map(&:to_i)
L = []
R = []
LA = []
RA = []
LL = Array.new(N, -1)
RR = Array.new(N, -1)

min_v = Float::INFINITY
0.upto(N - 1) do |i|
  a = A[i]
  idx = LA.bsearch_index { |x| x > a } || LA.size
  if idx < LA.size
    LL[i] = a + LA[idx]
  end
  LA.insert(idx, a)

  if min_v > a
    min_v = a
  end

  L[i] = min_v
end

min_v = Float::INFINITY
(N - 1).downto(0) do |i|
  a = A[i]
  idx = RA.bsearch_index { |x| x > a } || RA.size
  if idx < RA.size
    RR[i] = a + RA[idx]
  end
  RA.insert(idx, a)

  if min_v > a
    min_v = a
  end
  R[i] = min_v
end

ans = Float::INFINITY

1.upto(N - 2) do |i|
  a = A[i]
  l = LL[i]
  r = RR[i]
  next if l == -1
  next if r == -1

  v = l + r - a

  ans = v if ans > v
end

1.upto(N - 2) do |i|
  a = A[i]
  l = L[i - 1]
  r = R[i + 1]
  next if l > a
  next if r > a

  v = a + l + r
  ans = v if ans > v
end

if ans == Float::INFINITY
  puts -1
else
  puts ans
end