def recall(y_hat, y, cls):
    """
    Function to calculate the recall

    Inputs:
    > y_hat: pd.Series of predictions
    > y: pd.Series of ground truth
    > cls: The class chosen
    Output:
    > Returns the recall as float
    """
    assert(y_hat.size == y.size)
    
    num = 0
    den = 0
    for i in range(len(y)):
        if y[i] == y_hat[i] == cls:
            num+=1
        if y[i] == cls:
            den+=1
    if den == 0: return None
    return round(100*num/den, 2)