Please help FPS Sprint Script

Please help sprint script :

    if(GetKeyDown("left Shift"))
    {
    //What i write here ?
    }

moveSpeed*=2;

ok but wot do you type to make it go back when you release

because the answers not:

if(GetKeyDown(“left Shift”) = false) { moveSpeed*=1 }

also if u klick it 5 times it will just keep multiplying