2001-1998+1995-1992+… +15-12+9-6+3．

2001-1998+1995-1992+… +15-12+9-6+3．

（2）2001-1998+1995-1992+… +15-12+9-6+3，=（2001-1998）+（1995-1992）+… +（15-12）+（9-6）+3，=[（2001÷3-1）÷2]×3+3，=[666÷2]×3+3，=333×3+3，=1002．

An idiom in which two idioms are connected
For example, the outside of the gold and jade, the inside

It's a long way to go. The city gate is on fire. It's a disaster for the fish to lay down their butcher's knife and become a Buddha. The red one who is close to the red one, the black one who is close to the ink one foot high, and the devil one foot high. Xiang Zhuang dances his sword. He intends to fight with the snipe and the clam for the benefit of the fisherman, but not enter the tiger's den

How much is one plus one
The answer is an idiom

If it's an idiom, it's no three no four