input: Write a python function to implement a stack with push, pop, and peek operations.
  assert stack.push(1) == None
  assert stack.push(2) == None
  assert stack.pop() == 2
  assert stack.peek() == 1
  assert stack.pop() == 1
  assert stack.pop() == -1
output: class Stack:
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          if not self.items:
              return -1
          return self.items.pop()
  
      def peek(self):
          if not self.items:
              return -1
          return self.items[-1]
  
  stack = Stack()