calc

gistfile1.cs

using System;
using System.Text.RegularExpressions;
using System.Linq;

namespace fun {
    class MainClass {
        public static void Main (string[] args) {
            var input = Console.ReadLine ();
            Console.WriteLine (input.Contains ("+") ? Enumerable.Aggregate (from n in (new Regex (@"\d+|\.\d+|\d+\.").Matches (input).OfType<Match> ())select (n.ToString ()), (acc, x) => (float.Parse (acc) + float.Parse (x)).ToString()) : "");
        }
    }
}

// Tests

> 2 + 3
5

> .1 + 1 + 1.
2.1

> 111.111 + 222.222 + 333.333
666.666