Quadratic equation in Dart

import 'dart:io';
import 'dart:math' show sqrt;

int readInt() {
  String input = stdin.readLineSync();
  return int.parse(input);
}

main() {
  int A, B, C;
  try {
    A = readInt();
    B = readInt();
    C = readInt();
  }
  on FormatException {
    print("Coefficient is not a number.");
    return;
  }
  if (A == 0) {
    print("Not a quadratic equation.");
    return;
  }
  int D = B * B - 4 * A * C;
  double p1 = - B / 2.0 / A;
  double p2 = sqrt(D.abs()) / 2.0 / A;
  if (D == 0) {
    print("x = $p1");
  } else {
    if (D > 0) {
      print("x1 = ${p1 + p2}");
      print("x2 = ${p1 - p2}");
    } else {
      print("x1 = ($p1, $p2)");
      print("x2 = ($p1, ${-p2})");
    }
  }
}