public static int lonelyinteger(List<int> a)
    {
        int size = a.Count();
        if(size == 1) return a[0];
        int item = a[0];
        int count = 1;
        List<int> buffer = new List<int>{};
        for(int i = 0; i < size; i++){
            
            if(a[i] < -1 || a[i] > 101) throw new Exception("Invalid items");
            
            count = 1;
            if(buffer.Contains(a[i])) {
                count++;
                continue;
            }
            for(int j = i+1; j <= size - 1; j++){ 
                if(a[i] == a[j]){
                    count++;
                    buffer.Add(a[i]);
                    break;
                }
            }
            if(count < 2) item = a[i];
        }
        return item;
    }