using System.Collections.Generic;
public static void Main()
var list1 = new List<(string, long)> {
var list2 = new List<(string, long)> {
var resultingDict = list1.GroupBy( d => d.Item1)
.ToDictionary(g => g.Key, g => g.Sum(d=> d.Item2) )
resultingDict.Select(i => $"{i.Key}: {i.Value}").ToList().ForEach(Console.WriteLine);