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public static string Multiply(int a, int b) {
	StringBuilder sb = new StringBuilder ();
	int aLength = a.ToString ().Length;
	int bLength = b.ToString ().Length;
	sb.AppendLine (a + "*" + b);
	sb.AppendLine (new string ('-', aLength + bLength + 1));
	int endPos = aLength + 2;
	foreach (char c in b.ToString()) {
		int tmp = a * (c - '0');
		int tmpLength = tmp.ToString ().Length;
		sb.AppendLine (new string (' ', endPos - tmpLength) + tmp);
		endPos++;
	}
	sb.AppendLine (new string ('-', aLength + bLength + 1));
	sb.AppendLine ((a * b).ToString ().PadLeft (aLength + bLength + 1, ' '));
	return sb.ToString ();
}

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18*23
-----
  36
   54
-----
  414

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public static String padLeft(String s, int n) {
	return String.format("%1$" + n + "s", s);
}

public static String multiply(int a, int b) {
	StringBuilder sb = new StringBuilder();
	int aLength = String.format("%d", a).length();
	int bLength = String.format("%d", b).length();
	sb.append(a + "*" + b + "\n");
	sb.append(padLeft("", aLength + bLength + 1).replace(' ', '-') + "\n");
	int endPos = aLength + 2;
	String bStr = String.format("%d", b);
	for (int i = 0; i < bLength; i++) {
		int tmp = a * (bStr.charAt(i) - '0');
		String tmpStr = String.format("%d", tmp);
		sb.append(padLeft(tmpStr, endPos) + "\n");
		endPos++;
	}
	sb.append(padLeft("", aLength + bLength + 1).replace(' ', '-') + "\n");
	sb.append(padLeft(String.format("%d", a * b), aLength + bLength + 1)
			+ "\n");
	return sb.toString();
}