cable cable cable

Connecting the display screen and signal sources which produce different color signals by cables, then the display screen can show the color of the signal source.Notice that every signal source can only send signals to one display screen each time.
Now you have $M$ display screens and $K$ different signal sources($K \leq M \leq 2^{32}-1$). Select $K$ display screens from $M$ display screens, how many cables are needed at least so that **any** $K$ display screens you select can show exactly $K$ different colors.

Multiple cases (no more than $100$), for each test case:
there is one line contains two integers $M$ and $K$.

Output the minimum number of cables $N$.

3 2
20 15

4
90

As the picture is shown, when you select M1 and M2, M1 show the color of K1, and M2 show the color of K2.

When you select M3 and M2, M2 show the color of K1 and M3 show the color of K2.

When you select M1 and M3, M1 show the color of K1.

2017 ACM/ICPC Asia Regional Shenyang Online