class expStack:
    def __init__(self):
        self.items = list()
    #---------------------------
    def isEmpty(self):
        return len(self.items) == 0
    #---------------------------
    def length(self):
        return len(self.items)
    #------------------------------
    def push(self, element):
        self.items.append(element)
    #-----------------------------
    def top(self):
        if not self.isEmpty():
             return self.items[len(self.items) - 1]
    #-----------------------------
    def pop(self):
        if not self.isEmpty():
            return self.items.pop()
    #-------------------------