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// Modular Exponentiation
// Compute x^n mod m
int modexp(int x, int n, int m) {
  if (n == 0) return 1;
  if (n & 1) return ((x % m) * modexp(x, n - 1, m)) % m;
  int y = modexp(x, n / 2, m);
  return (y * y) % m;
}