债券久期计算

def solve(____):
    period_rate = ytm / freq
    periods = maturity * freq
    coupon = face_value * coupon_rate / freq
    price = ____
    weighted_sum = ____
    for t in range(____, periods + ____):
        cf = coupon if t < periods else coupon + face_value
        pv = cf / (____ + period_rate) ** t
        price += pv
        weighted_sum += t * pv
    duration = weighted_sum / price / freq
    return round(____)