ค้นหาเส้นทางจากสหรัฐอเมริกาไปแคนาดา
Find the number of routes from the United States to Canada.
df = pd.DataFrame(columns=["dst_apid", "src_apid", "apid", "country"])
df[(df['dst_apid'].isin(df[df['country']=='Canada']['apid'])) & (df['src_apid'].isin(df[df['country']=='United States']['apid']))].shape[0]