`M = 1 . 1! + 2 . 2! + 3 . 3! + 4 . 4! + 5 . 5!`
`M = 1 . 1 + 2 . 1 . 2 + 3 . 1 . 2 . 3 + 4 . 1 . 2 . 3 . 4 + 5 . 1 . 2 . 3 . 4 . 5`
`M = 2 + 4 + 18 + 96 + 600`
`M = 6 + 18 + 96 + 600`
`M = 24 + 96 + 600`
`M = 120 + 600`
`M = 720`
$\text{Vậy}$ `M = 720`
