a = int(input("Enter first number: "))
b = int(input("Enter second number: "))

print("1.Add 2.Sub 3.Mul 4.Div")
choice = int(input("Enter choice: "))

if choice == 1:
    print(a + b)
elif choice == 2:
    print(a - b)
elif choice == 3:
    print(a * b)
elif choice == 4:
    print(a / b)
else:
    print("Invalid choice")

Enter first number:  21
Enter second number:  21


1.Add 2.Sub 3.Mul 4.Div


Enter choice:  3


441

num = int(input("Enter number: "))

if num > 0:
    print("Positive")
elif num < 0:
    print("Negative")
else:
    print("Zero")

Enter number:  4


Positive

n = int(input("Enter n: "))
sum = 0

for i in range(1, n+1):
    sum += i

print("Sum =", sum)

Enter n:  6


Sum = 21