Strong Duality and Duals of linear programming problem

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I have the following problem: $ max_{x,y} \ x + y $

subject to

$ 2x + y \leq 1 $

$ x + 3y \leq 3 $

$ x,y \geq 0 $

How to find the dual of this problem using the Lagrangian?

I have done the following steps:

Step 1: Rewriting the problem as a minimization

$ - \ min_{x,y} \ -x - y $

subject to

$ 2x + y \leq 1 $

$ x + 3y \leq 3 $

$ x,y \geq 0 $

Step 2: Formulating the Lagrangian

$ L(x,\lambda) = -x - y + \lambda_1(2x+y-2) + \lambda_2(x + 3y -3) - \lambda_3 x - \lambda_4 y \ \ \ \ \ \ \ \ \ \lambda_i \geq 0 $

Step 3: In order to find the dual I have to minimize the lagrangian

I am stuck in this step. How do I proceed?