using System;

using System.Collections.Generic;

using System.Linq;

using System.Globalization;

public class Program {

    public static void Main() {

        var dic = new Dictionary<String, IEnumerable<String>> {

            {"a", new[] { "b", "c", "d" }},

            {"e", new[] { "f", "g", "h" }},

            {"i", new[] { "j", "k", "l" }}

        };

        var r = dic.SelectMany((x, i) => x.Value.Select((y, j) => new KeyValuePair<String, String>($"{(i * x.Value.Count() + j).ToString()}_{x.Key}", y)));

        r.ToList().ForEach(x => { Console.WriteLine(x); });

    }

}