29. From One Corner to Another

I'm a slow walker, but I never walk backwards.

Task Description

Write a program to compute the number of ways to go from the lower left corner of a matrix to upper right corner. The matrix has $r$ rows and $c$ columns. You can only move one cell to another cell, and only to the right or to the top. Please compute the number of ways you can go. Note that you are strongly encouraged to think in "recursion" to solve this problem.

Limits

Both $r$ and $c$ are no more than $12$.

Input Format

There are one line in the input. The first line has $r$ and $c$.

Output Format

There are one lines in the output. The first line has the number of ways as in the description.

Sample Input

4 3

Sample Output

10

Discussion