using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string input = "a+aa-b";
		string variable = "a";
		string replacement = "1";
		Regex regex = new Regex(@"\b" + variable + @"\b");
		string result = regex.Replace(input, replacement);
		Console.WriteLine(result);
	}
}