N = gets.to_i
A = gets.split.map(&:to_i)
L = []
R = []

min_v = Float::INFINITY
0.upto(N - 1) do |i|
  a = A[i]
  min_v = a if min_v > a
  L[i] = min_v
end

min_v = Float::INFINITY
(N - 1).downto(0) do |i|
  a = A[i]
  min_v = a if min_v > a
  R[i] = min_v
end

ans = Float::INFINITY

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