1,234
6
123456
123,456
8
12345678
12,345,678
'''
# a=int(input())
# b=input()
# stack=[]
# c = len(b)
# cnt=0
# m=""
# for i in range(c-1,-1,-1):
# stack.append(b[i])
# cnt+=1
# if cnt%3==0 :
# stack.append(",")
# if stack[-1]==",":
# stack.pop()
# stack.reverse()
# for j in range(len(stack)):
# print(stack[j],end='')
#
# a = int(input())
#
# for i in range(a) :
# print(i,end=' ')
# print()
# for i in range(a,0,-1) :
# print(i, end=' ')
# print()
# for i in range(a-1,-1,-1) :
# print(i, end=' ')
a=int(input())
stack=[]
for i in range(a):
b=input()
if b=="top()":
if len(stack)==0:
print(-1)
else:
print(stack[-1])
elif b=="pop()":
if len(stack)!=0:
stack.pop()
elif b=="size()":
print(len(stack))
elif b=="empty()":
if len(stack)==0:
print("true")
else :
print("false")
else:
x=b.split()
stack.append(int(x[1]))