ppois(3, 5)*ppois(2, 5)
## 0.03303602

1-sum(dpois(4:17,5)))*ppois(2,5)
## 0.03303669

1 - sum(dpois(12:29,20))
## 0.04320504

1-sum(dpois(12:29,10))
## 0.6967764