Permutation in String

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 s1 = Console.ReadLine();
string s2 = Console.ReadLine();
var need = new int[26], have = new int[26];
foreach (var c in s1) need[c - 'a']++;
int matches = 0;
for (int i = 0; i < 26; i++) if (need[i] == 0) matches++;
bool found = false;
for (int i = 0; i < s2.Length; i++) {
    int idx = s2[i] - 'a';
    have[idx]++;
    if (have[idx] == need[idx]) matches++;
    else if (have[idx] == need[idx] + 1) matches--;
    if (i >= s1.Length) {
        int outIdx = s2[i - s1.Length] - 'a';
        have[outIdx]--;
        if (have[outIdx] == need[outIdx]) matches++;
        else if (have[outIdx] == need[outIdx] - 1) matches--;
    }
    if (matches == 26) { found = true; break; }
}
Wb(found);
    }
}