from collections import deque

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

X=deque()
for a in A:
    if a=="+":
        q=X.pop()
        p=X.pop()
        X.append(p+q)
    elif a=="-":
        q=X.pop()
        p=X.pop()
        X.append(p-q)
    else:
        X.append(int(a))
print(X[0])