1.2
l = int(input())
s = list(map(int, input().split()))
 
tup = [0]
tr2 = [0] 
 
for i in range(l):
    while tup[-1] == tr2[-1] + 1:
        tr2.append(tup[-1])
        tup.pop()
    if s[i] == tr2[-1] + 1:
        tr2.append(s[i])
    else:
        tup.append(s[i])
 
while tup[-1] == tr2[-1] + 1:
    tr2.append(tup[-1])
    tup.pop()
 
if tr2[-1] == l:
    print('YES')
else:
    print('NO')
    

1.4
expression = input()
stack = []
incorrect_count = 0

for char in expression:
    if char == '(':
        stack.append(char)
    elif char == ')':
        if stack:
            stack.pop()
        else:
            incorrect_count += 1

incorrect_count += len(stack)

if incorrect_count == 0:
    print("Yes")
else:
  
    print("No. Incorrect brackets =",  incorrect_count)
    
    
1.5
st = input().split()
stack = []
for x in st:
	if x in "+-*/":
		op2 = int(stack.pop())
		op1 = int(stack.pop())
		if x == "+": res = op1 + op2
		elif x == "-": res = op1 - op2
		elif x == "*": res = op1 * op2
		else: res = op1 // op2
		stack.append ( res )
	else:
		stack.append ( x )
print (stack[0])