dataframe添加统计数量 求解

原始数据
发送人 接收人
A B
A B
A C
A D

理想输出数据
发送人 接收人 次数
A B 2
A B 2
A C 1
A D 1

想统计出A给B发了多少次， A给C发了多少次这样

df = pandas.DataFrame([{"send": "A", "receive":"B", "t":1},{"send": "A", "receive":"B", "t":2},{"send": "A", "receive":"C", "t":3},{"send": "A", "receive":"D", "t":4}])
df.groupby(["send", "receive"]).count()