column of numbers: 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5, ..., in which the natural number n appears n times. Then, what is the remainder of dividing the 1999th number in this column by 5?
一列数：1，2，2，3，3，3，4，4，4，4，5，5，5，5，5，…，其中自然数n出现n次．那么，这列数中的第1999个数除以5的余数是多少．
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