a question related with morse theory

Question

Show that there exists no smooth function $f:\mathbb{R}^2→\mathbb{R}$,such that $f(x,y)\geq 0$ for any $(x,y)\in\mathbb{R}^2$, with exactly two critical points$(x_1,y_1)\in\mathbb{R}^2$, $(x_2,y_2)\in\mathbb{R}^2$ and $f(x_1,y_1 )=f(x_2,y_2 )=0$.