# 3. Option 1.

Vasya calculated two sums: the sum of consecutive odd numbers from 1 to 2021: $(1+3+5+\ldots+2021)$ and the sum of consecutive even numbers from 2 to 2020: $(2+4+6+\ldots+2020)$. After that, he subtracted the smaller sum from the larger one. What result did he get?
Answer: 1011.

Solution. Let's find the difference between the sums of odd and even numbers: $(1+3+5+\ldots+2021)-(2+4+6+\ldots+2020)=(2021-2020)+(2019-2018)+\ldots+(3-2)+1=1011$.