Factorial !

An exclamation mark following a number denotes what is called a factorial. This notation is used as shorthand for a number shouting loud (not really) by multiplying itself with all the positive integers smaller than itself. For example, $4!=4\times 3\times 2\times 1=24.$ In general, for any positive integer $n$, its factorial is defined as $n!=n\times (n-1)\times (n-2)\times \dots \times 1.$