How to ask user to input an equation (could be mulitvariable)

I am setting up a matlab code that asks for input of an equation from the user to minimize. This equation could be a multivariable equation (x,y,z..). so far I have equation= input('Enter an equation to minimize: ','s');

Eventually we will take the gradient and hessian.

[Mikhail]

So what is the problem? you ask user to input equation and write it to your string. After that you must decode this equation.

[Siam]

As Mikhail said, set the input option from the user as a string and then decode the equation based on the variables for your input equation.