1 + 2 + 3 +,,,,,,,,,,,,,,,,, + 99 + 1000 + 1001

1 + 2 + 3 +,,,,,,,,,,,,,,,,, + 99 + 1000 + 1001

1 + 2 + 3 +...+ 99 + 1000 + 1001
= (1 + 99) + (2 + 98) +...+ (44 + 46) + 45 + 1000 + 1001
= 44 * 100 + 45 + 2001
= 4445 + 2001
= 6446