H, W = gets.split.map(&:to_i)
A = (H - 2).times.map { gets.split.map(&:to_i) }

dp = Array.new(W) { Array.new(H, Float::INFINITY) }

(H - 2).times do |y|
  next if A[y][0] == -1

  dp[0][y] = A[y][0]
end

1.upto(W - 1) do |w|
  (H - 2).times do |y|
    next if A[y][w] == -1

    if y - 1 >= 0 && dp[w][y] > dp[w - 1][y - 1] + A[y][w]
      dp[w][y] = dp[w - 1][y - 1] + A[y][w]
    end

    if dp[w][y] > dp[w - 1][y] + A[y][w]
      dp[w][y] = dp[w - 1][y] + A[y][w]
    end

    if y + 1 < H - 2 && dp[w][y] > dp[w - 1][y + 1] + A[y][w]
      dp[w][y] = dp[w - 1][y + 1] + A[y][w]
    end
  end
end

if dp[W - 1].min == Float::INFINITY
  puts -1
else
  puts dp[W - 1].min
end