If you are multiplying two integers which have a difference between them as 2, please check the following steps to find the product of them

1.Get the square of the number between these 2 numbers

2.subtract 1 from the value got in step 1 & you get the answer.
Example : 11*13 = 12*12-1 = 144 -1 = 143

This is a very useful tip, if you remember the squares or you can find the squares quickly.

Another example: 249 *251 = 250*250 -1 = 62500 -1 = 62499

Another example: 899 *901 = 900*900 -1 = 810000 -1 = 809999

Another example: 3999 *4001 = 4000*4000 -1 = 16000000 -1 = 15999999