class SegmentTree
	def initialize(arr)
		@h = (arr.length - 1).bit_length
		@n = 2**@h
		@ie = Float::INFINITY
		@tree = Array.new(2*@n, @ie)
		0.upto(arr.length-1) {|i|
			@tree[@n + i] = arr[i]
		}
		(@n-1).downto(1) {|i|
			@tree[i] = [@tree[2 * i], @tree[2 * i + 1]].min
		}
	end
	def set(idx, x)
		idx += @n
		@tree[idx] = x
		while idx > 0 do
			idx >>= 1
			@tree[idx] = [@tree[2 * idx], @tree[2 * idx + 1]].min
		end
	end
	def query(lt, rt)
		lt += @n
		rt += @n
		vl = vr = @ie
		while rt - lt > 0 do
			if lt & 1 != 0 then
				vl = [vl, @tree[lt]].min
				lt += 1
			end
			if rt & 1 != 0 then
				rt -= 1
				vr = [@tree[rt], vr].min
			end
			lt >>= 1
			rt >>= 1
		end
		[vl, vr].min
	end
end

INF = Float::INFINITY

N = gets.to_i
A = gets.split(" ").map{|s| s.to_i}

S = A.map.with_index{|x, i| [x, i]}.sort!

res = INF

st1 = SegmentTree.new([INF] * N)

0.upto(N-1) {|i|
	a, idx = S[i]
	st1.set(idx, a)
	lt = st1.query(0, idx)
	rt = st1.query(idx + 1, N)
	next if lt == INF or rt == INF
	res = [res, lt + rt + a].min
}

st2 = SegmentTree.new([INF] * N)

(N-1).downto(0) {|i|
	a, idx = S[i]
	st2.set(idx, a)
	lt = st2.query(0, idx)
	rt = st2.query(idx + 1, N)
	next if lt == INF or rt == INF
	res = [res, lt + rt + a].min
}

puts res != INF ? res : -1