Extrema on a circle

Let $f:\mathbb{R}^2\to\mathbb{R}$ be the function defined by $f(x, y)=x^2-12y$. If $M$ and $m$ be the maximum value and the minimum value, respectively, of the function $f$ on the circle $x^2+y^2=49$, then $|M|+|m|$ equals __________