v.
sum(a1:b1)
sum(a2:b2)
sum(1!d1:d2) + sum(d1:d2)
1!B2
sum(1!d1:d2) →
sum(1!d1:d2)
sum(1!a1:b2) + sum(2!a1:b2) →
sum(1!a1:b2) + sum(2!a1:b2)