N = gets.to_i
A = gets.chomp.split

stack = []
A.each do |a|
  if a == '+'
    u, v = stack.pop(2)
    stack << u + v
  elsif a == '-'
    u, v = stack.pop(2)
    stack << u - v
  else
    stack << a.to_i
  end
end

puts stack.first