N = int(input())
A = list(input().split())

q = []
for i in A:
    if i == '+':
        a = q.pop()
        b = q.pop()
        q.append(a+b)
    elif i == '-':
        a = q.pop()
        b = q.pop()
        q.append(b-a)
    else:
        q.append(int(i))
print(q.pop())