Three letters are dictated to three persons and an envelope is addressed to each of them, the letters are inserted into the envelopes at random so that each envelope contains exactly one letter. Find the probability that at least one letter is in its proper envelope.

Explanation

Let `L_1, L_2, L_3` be three letters and `E_1, E_2, E_3` be their corresponding envelopes respectively.


There are 6 ways of inserting 3 letters in envelops as follows:


`L_1 E_1, L_2 E_3, L_3 E_2`


`L_1 E_1, L_2 E_2, L_3 E_3`


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