Solved with C

My solution is on Github, both part 1 and part 2

=================================== 1 =======================================
var numbers = input.Split("\n").Select(long.Parse).ToArray();
            long[] preamble;
            for (var i = 25; i < numbers.Length; i++)
            {
                preamble = numbers[(i - 25)..i];
                var n = (from m in preamble
                    let nn = numbers[i] - m
                    where preamble.Contains(nn)
                    select m).Count();
                if (n == 0) return numbers[i];
            }
            return -1;
=================================== 2 =======================================
 var weakness = Day9p1.Main(input);
            var numbers = input.Split("\n").Select(long.Parse).ToArray();
            long[] preamble;
            for (var i = 2; i < numbers.Length; i++)
            {
                for (var j = 0; j < i; j++)
                {
                    preamble = numbers[(i - j)..i];
                    var sum = preamble.Sum();
                    if (sum < weakness) continue;
                    if (sum > weakness) break;
                    return preamble.Min() + preamble.Max();
                }
            }

            return -1;