using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Linq;
public class Test
{
	public static void Main()
	{
		var pat = @"(?m)(?:(\A)|^(?!\A))(.*\b\s+(\d+)\r?\n)";
		var s = @"10 25
32 44
56 88
102 127
135 14510 25
32 44
56 88
102 127
135 145";
		var res = Regex.Replace(s, pat, m => m.Groups[1].Success ? 
			$"0 {m.Groups[2].Value}{m.Groups[3].Value} " : $"{m.Groups[2].Value}{m.Groups[3].Value} ");
		Console.WriteLine(res);
	}
}