如何根据R中其他变量的类别汇总值

我有一个数据集，它显示了甲方和乙方在X国的宗教信仰，以及每个国家的宗教信徒的百分比.

df <- data.frame(
  PartyA = c("Christian","Muslim","Muslim","Jewish","Sikh"),
  PartyB = c("Jewish","Muslim","Christian","Muslim","Buddhist"),
  ChristianPop = c(12,1,74,14,17),
  MuslimPop = c(71,93,5,86,13),
  JewishPop = c(9,2,12,0,4),
  SikhPop = c(0,0,1,0,10),
  BuddhistPop = c(1,0,2,0,45)
)
#      PartyA    PartyB ChristianPop MuslimPop JewishPop SikhPop BuddhistPop
# 1 Christian    Jewish           12        71         9       0           1
# 2    Muslim    Muslim            1        93         2       0           0
# 3    Muslim Christian           74         5        12       1           2
# 4    Jewish    Muslim           14        86         0       0           0
# 5      Sikh  Buddhist           17        13         4      10          45

说到这里，我想把"涉足"的宗教信徒的总数加在一起.因此，第一行的变量等于12+9，第二行的变量只有93(因为A方和B方相同，所以没有添加)，依此类推.

#      PartyA    PartyB ChristianPop MuslimPop JewishPop SikhPop BuddhistPop PartyRel
# 1 Christian    Jewish           12        71         9       0           1       21
# 2    Muslim    Muslim            1        93         2       0           0       93
# 3    Muslim Christian           74         5        12       1           2       79
# 4    Jewish    Muslim           14        86         0       0           0       86
# 5      Sikh  Buddhist           17        13         4      10          45       55

我甚至很难找到从哪里开始，我会非常感激你的帮助.