Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What's the sum of the number of episodes that originally aired after 1991 with a series number smaller than 21 and a DVD Region 2 release date of 26 march 2012?

### Inputs:
table_name: table, columns: ['Series Number', 'Number of Episodes', 'Original Air Date', 'DVD Region 2 release date', 'DVD Region 1 release date']

### Response:
SELECT SUM Number of Episodes FROM table WHERE Original Air Date > 1991 AND Series Number < 21 AND DVD Region 2 release date = 26 march 2012