Sprint Error script?

I just made a script for sprinting:

//The Sprint script
if( Input.GetButtonDown("Shift")) {
maxForwardSpeed = maxSprintSpeed ;
maxSidewaysSpeed = maxSprintSpeed ;
maxBackwardSpeed = "0" ;

}
if (Input.GetButtonUp(“Shift”)) {
maxForwardSpeed = maxForwardSpeed ;
maxSidewaysSpeed = maxSidewaysSpeed ;
maxBackwardSpeed = maxBackwardSpeed ;
}
What is the problem with this script?

this:

maxForwardSpeed = maxForwardSpeed ;

doesn’t make any sense.

I imagine maxBackwardSpeed as an Integer variable. If so, you have to declare it as

maxBackwardSpeed = 0;

and not as

maxBackwardSpeed = "0";