N=int(input())
stack=[]
for a in input().split():
    if a=="+":
        u=stack.pop()
        v=stack.pop()
        stack.append(v+u)
    elif a=='-':
        u=stack.pop()
        v=stack.pop()
        stack.append(v-u)
    else:
        stack.append(int(a))
print(stack.pop())