int freq[26];
    
    memset(freq,0,sizeof(freq));
    
    for(auto r:s)
        freq[r-'a']++;
    
    for(auto s:t)
        freq[s-'a']--;
    
    for(int i=0;i<26;i++)
        if(freq[i]==-1)
            return 'a'+i;
    
    
    
    
    return 'a';