GTIN 8, 12, 13, 14

``` csharp
            var sum = 0;
            var multiply = false;
            foreach (var c in s.ToCharArray()) 
            {
                var n = int.Parse(c.ToString());
                sum += multiply ? (n*3) : n;
                multiply = !multiply;
            }  
            var mod = (sum % 10);
            if (mod == 0)
                return 0;
            return 10 - mod;
```
