If the consumer wants to buy \({x_1}\) units of item 1 and \({x_2}\) units of item 2 and the prices of the two items are \({p_1}\)and \({p_2}\) respectively, and M is the income of the consumer, then \({p_{1x1}}\) + \({p_{2x2}}\) ≤ M is called the consumer's _______ .