The probability that if a 12- element sequence of integers ( $a_1,a_2,\mathrm{.....},a_{12}$ ) (not necessarily distinct) is selected from {1, 2, 3, ……, 50} satisfies the condition, $1\le a_1\le a_2\mathrm{.....}\le a_{11}\le a_{12}\le 50$  is