There are 1987 numbers written on the blackboard: 1, 2, 3, ..., 1986, 1987. Randomly erasing several numbers and adding the remainder of the sum of the erased numbers divided by 7 is called an operation. If after several times of this operation, there are only two numbers left on the blackboard, one is 987, then what is the other number?
黑板上写有1987个数：1，2，3，…，1986，1987．任意擦去若干个数，并添上被擦去的这些数的和被7除的余数，称为一个操作．如果经过若干次这种操作，黑板上只剩下了两个数，一个是987，那么，另一个数是多少．
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