n = read_line.to_i
a = read_line.split
stack = Deque(Int32).new
a.each do |i|
  if i == "+"
    b, a = stack.pop, stack.pop
    stack.push(a + b)
  elsif i == "-"
    b, a = stack.pop, stack.pop
    stack.push(a - b)
  else
    stack.push(i.to_i)
  end
end
puts stack.last