Minimum Window Substring

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class Program
{
    static int[] Ria(string line = null)
    {
        line ??= Console.ReadLine();
        if (string.IsNullOrWhiteSpace(line)) return Array.Empty<int>();
        return line.Trim().Split(new[] { ' ', ',', '\t' }, StringSplitOptions.RemoveEmptyEntries)
            .Select(int.Parse).ToArray();
    }
    static string[] Rsa()
    {
        int n = int.Parse(Console.ReadLine());
        var arr = new string[n];
        for (int i = 0; i < n; i++) arr[i] = Console.ReadLine();
        return arr;
    }
    static void W(params object[] parts) => Console.WriteLine(string.Join(" ", parts));
    static void Wb(bool v) => Console.WriteLine(v ? "true" : "false");
    static void Wi(int v) => Console.WriteLine(v);
    static void Ws(string v) => Console.WriteLine(v);

    static void Main()
    {
string s = Console.ReadLine();
string t = Console.ReadLine();
var need = new Dictionary<char, int>();
foreach (var c in t) need[c] = need.GetValueOrDefault(c) + 1;
int have = 0, goal = need.Count, start = 0, bestLen = int.MaxValue, bestStart = 0;
var window = new Dictionary<char, int>();
for (int end = 0; end < s.Length; end++) {
    char c = s[end];
    window[c] = window.GetValueOrDefault(c) + 1;
    if (need.ContainsKey(c) && window[c] == need[c]) have++;
    while (have == goal) {
        if (end - start + 1 < bestLen) { bestLen = end - start + 1; bestStart = start; }
        char left = s[start++];
        window[left]--;
        if (need.ContainsKey(left) && window[left] < need[left]) have--;
    }
}
Ws(bestLen == int.MaxValue ? "" : s.Substring(bestStart, bestLen));
    }
}