Erase two numbers at will and subtract one from the sum of these two numbers.

There are 99 trees left, and their sum is 5050- 1=5049.

Each operation is reduced by one number on the blackboard, and the total reduction is 1.

When there is one number left at last, this number is reduced by 99, which means that the operation is 99 times, and the total is reduced by 99.

The sum at this time is 5050-99=495 1,

Explain that the last remaining number is 495 1.