Karatsuba for dart (Recursion function)

Question

import 'dart:math';

int karatsuba(x, y) {
  var n, m, a, b, c, d, p, q, ac, bd, pq, adcb;
  n = max(2, 2);
  m = (n ~/ 2);

  a = (x / pow(10, m)).floor(); // 56
  b = (x % pow(10, m)); // 78
  c = (y / pow(10, m)).floor(); // 12
  d = (y % pow(10, m)); // 34

  p = a + b;
  q = c + d;

  ac = karatsuba(a, c); // problem
  bd = karatsuba(b, d);
  pq = karatsuba(p, q);

  adcb = pq - ac - bd;

  return ac * (pow(10, n)) + adcb * (pow(10, m)) + bd;
}

void main() {
  karatsuba(12, 12);
}

ac = karatsuba(a, c) => here my problem. I don't understand why dart making endless loop for this. It is work for python but don't work from dart.