Whats a good sprinting script?

For a long time iv’e been wondering what is a good JavaScript for making my character sprint holding left sprint? I am making a slender game and almost everyone knows that you cant escape from slender man quick enough without sprint. No crouch, because that is just weird since it will only make you slower. May someone out there be kind enough to write a script and paste it on this question? I walk at 4 miles per hour in the game and I want to sprint at 6 miles per hour. May someone please help me out with this?

Hi I have answered a similar question here

by the way are you using character controller or a rigidbody the above one is based on character controller if you want rigidbody please do feel free to ask