N = int(input())
A = input().split()
stack = []

for a in A:
    if a == '+':
        stack.insert(0, stack.pop(0) + stack.pop(0))
    elif a == '-':
        stack.insert(0, stack.pop(0) - stack.pop(0))
    else:
        stack.append(int(a))

print(stack[0])