using System.Text;

public class Solution
{
    public string GenerateString(string str1, string str2)
    {
        var len = str1.Length + str2.Length - 1;
        var answer = new char[len];

        // Сначала обработать все T
        for (var i = 0; i < str1.Length; i++)
        {
            if (str1[i] == 'T')
            {
                var sbIdx = i;
                for (var j = 0; j < str2.Length; j++)
                {
                    if (answer[sbIdx] != 0 && answer[sbIdx] != str2[j])
                        return "";
                    answer[sbIdx++] = str2[j];
                }
            }
        }
        // Теперь обработать все F
        var canChange = new bool[len];
        var lastF = -1;
        for (var i = 0; i < str1.Length; i++)
            if (str1[i] == 'F')
            {
                if (ProcessF(answer.AsSpan()[i..(i + str2.Length)], str2, canChange.AsSpan()[i..(i + str2.Length)], out var goBack) == false)
                    return "";
                if(goBack && lastF != -1) 
                    i = lastF - 1;
                else
                    lastF = i;
            }

        return new string(answer);
    }

    private bool ProcessF(Span<char> chars, string s2, Span<bool> canChange, out bool goBack)
    {
        goBack = false;

        var equal = true;
        var emptyCount = 0;
        var hasDiff = false;
        var lastCanChange = -1;
        for (var i = 0; i < s2.Length; i++)
        {
            equal &= chars[i] == s2[i];
            if (chars[i] == 0)
                emptyCount++;
            else if (chars[i] != s2[i])
                hasDiff = true;
            if (canChange[i])
                lastCanChange = i;
        }
        if (equal)
        {
            if (lastCanChange >= 0)
            {
                chars[lastCanChange]++;
                if (chars[lastCanChange] == 'z' + 1)
                    chars[lastCanChange] = 'a';
                goBack = true;
                return true;
            }
            else
                return false;
        }
        for (var i = 0; i < chars.Length && emptyCount > 0; i++)
        {
            if (chars[i] != 0)
                continue;
            var insert = 'a';
            if (emptyCount == 1 && s2[i] == insert && !hasDiff)
                insert++;
            chars[i] = insert;
            canChange[i] = true;
            if (s2[i] != insert)
                hasDiff = true;
            emptyCount--;
        }
        return true;
    }
}