code: def mul_inv(a, b):
          
          b0 = b
          x0, x1 = 0, 1
          if b == 1:
              return 1
          while a > 1:
              q = a // b
              a, b = b, a % b
              x0, x1 = x1 - q * x0, x0
          if x1 < 0:
              x1 += b0
          return x1
docs: Modular inversion a mod b
          :param a:
          :param b:
          :return: