from typing import List


def build_ngram(sentence: str, N: int = 2) -> List[str]:
    """split the sentence into N-gram

    Args:
        sentence (str): the sentence input 
        N (int, optional): the n parameter. Defaults to 2
        
    Returns:    
        list(str): list of substrings
    """
    return [sentence[i:i+N] for i in range(len(sentence)-N+1)]