For each permutation a_{1}, a_{2}, . . . , a_{10}
of the integers 1,2,3,...,10, form the sum
a_{1}  a_{2} + a_{3}  a_{4} + a_{5}  a_{6} + a_{7}  a_{8} + a_{9}  a_{10}.
Find the average value of all such sums.


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