title: Python 3 TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'
body: <pre><code>#Import the module
  from math import sqrt
  
  #Using while loop statement to make the program not finish before the user close the program.
  while True:
  
  #Print out the introduction message, and get the input value to solve the quadratic equation.
      print("ax^2+bx+c=0의 꼴로 된 방정식을 풀 수 있습니다. a, b, c의 값을 차례대로 입력하세요.")
      a = input("a를 입력하세요 : ")
      b = input("b를 입력하세요 : ")
      c = input("c를 입력하세요 : ")
  
  #Define function that checks whether the input values are natural number or negative number
      def func_num(n):
          if n[0] == '-':
              n = -int(n[1:])
          else:
              n = int(n)
  
  #Execute the function for the input value a, b, c
      func_num(a); func_num(b); func_num(c);
  
  #This if statement chekcs whether the solution of the quadratic equation going to be real number or imaginary number.
      if b ** 2 &lt; 4*a*c:
          solution1 = ((sqrt((b ** 2)-(4*a*c)))-b) / (2*a)
          solution2 = (-(sqrt((b ** 2)-(4*a*c)))-b) / (2*a)
      else:
          square_root = sqrt( -(b**2 - 4*a*c) ) + 1j
          solution1 = ( (square_root)  - b  ) / (2*a)
          solution2 = ( -(square_root)  - b  ) / (2*a)
  
  #Prints out the solution of the quadratic equation.
      print("정답은 바로바로... {}, {} 이거다!".format(solution1, solution2))
  </code></pre>
  
  <p>but it gives an error (for any input a, b, c that are integer):</p>
  
  <pre><code>Python 3 TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'
  </code></pre>
  
  <p>I think the problem is related to func_num() (because that error occurs when I do the calculation with number and strings)</p>
  
  <p>But I still can't find which part is wrong.</p>