Basic calculator

import math

i = "Invalid"

class start():
    def run(self):
        try:
            print("1. Basic Problems")
            print("2. Pythagorean Theory")
            print("3. Squareroots")
            print("4. Multiply by an exponent")
            r = int(input("Choose: "))
            if r == 1:
                basicObj.run()
            if r == 2:
                pyObj.run()
            if r == 3:
                sqrtObj.run()
            if r == 4:
                expoObj.run()


startObj = start()


class pythagorean():
    def run(self):
        try:
            x = int(input("A: "))
            y = int(input("B: "))
            c = a**2*b**2
            print(c)
            startObj.run()
        except:
            print(i)
            startObj.run()


pyObj = pythagorean()


class squareroots():
    def run(self):
        try:
            x = int(input("Number: "))
            print(sqrt(x)
            startObj.run()
        except:
            print(i)
            startObj.run()


sqrtObj = squareroots()


class exponents():
    def run(self):
        try:
            x = int(input("Base: "))
            y = int(input("Exponent: "))
            print(x**y)
            startObj.run()
        except:
            print(i)
            startObj.run()


expoObj = exponents()


class basicProblems():
    def run(self):
        try:
            x = eval(input("Enter Problem: "))
            print(x)
            startObj.run()
        except:
            print(i)
            startObj.run()

basicObj = basicProblems()
startObj.run()