"""
 Modular Exponential.
 Modular exponentiation is a type of exponentiation performed over a modulus.
 For more explanation, please check
 https://en.wikipedia.org/wiki/Modular_exponentiation
"""

"""Calculate Modular Exponential."""


def modular_exponential(base: int, power: int, mod: int):
 """
 >>> modular_exponential(5, 0, 10)
 1
 >>> modular_exponential(2, 8, 7)
 4
 >>> modular_exponential(3, -2, 9)
 -1
 """

 if power < 0:
 return -1
 base %= mod
 result = 1

 while power > 0:
 if power & 1:
 result = (result * base) % mod
 power = power >> 1
 base = (base * base) % mod

 return result


def main():
 """Call Modular Exponential Function."""
 print(modular_exponential(3, 200, 13))


if __name__ == "__main__":
 import doctest

 doctest.testmod()

 main()