1 + 2/1 - 3/2 + 4/3 - 5/4 + . . . - n/m

public static double add(int n, int m) {
  double result = 0.0;
  for (double i = 1.0; i <= n; i += 1.0) {
    if (i % 2 == 0) {
      result += -i / m;
    } else {
      result += i / m;
    }
  }
  return result;
}
add(10, 2)