C# Linq Sum query on IDictionary

I have an IDictionary<string,int> containing a distinct list of categories and respective counts with data looking something like this:

Category  |  Count
-------------------
A         |    2
B         |    2
Z         |    2
A_B       |    1
A_B_C     |    5

I'm looking for a query that sums the count of categories that start with common letters/string, so from the above the results I'm looking for is as follows:

Category  |  Count
-------------------
A         |    8     <- (a sum of A, A_B, A_B_C)
B  开发者_如何学JAVA       |    2
Z         |    2
A_B       |    6     <- (a sum of A_B, A_B_C)
A_B_C     |    5

---

var d = new Dictionary<string, int>()
{
    { "A", 2 },
    { "B", 2 },
    { "Z", 2 },
    { "A_B", 1 },
    { "A_B_C", 5 },
};

var d2 = d.Select(kvp => new KeyValuePair<string, int>(
                kvp.Key,
                d.Where(k => k.Key.StartsWith(kvp.Key))
                 .Sum(k => k.Value)))
          .ToDictionary(kvp => kvp.Key, kvp => kvp.Value);

---

Dictionary<string, int> data = new Dictionary<string,int>();

data.Add("A", 2);
data.Add("B", 2);
data.Add("Z", 2);
data.Add("A_B", 1);
data.Add("A_B_C", 5);

data.Dump();

    var result = from d in data
                 select new {Key = d.Key, Sum = (from dt in data where dt.Key.StartsWith(d.Key) select dt).Sum(x=>x.Value)};

result.Dump();

Linqpad code