Payoff range of a long 5 short 10 call spread

A stock is at $7$. You buy a call struck at $5$ and sell a call struck at $10$ with the same expiry. With $M$ the largest and $m$ the smallest possible payoff at expiry, compute $M^2+m^2$.

$25$. The spread pays nothing below $5$, rises one-for-one between $5$ and $10$, and is capped at $10-5=5$ above, so $M=5$, $m=0$.