Execution time limit is 1 second Runtime memory usage limit is 128 megabytes The function is given with nonnegative integer arguments m and n (m≤n):
f(m,n)=⎩⎨⎧1,m=01,m=nf(m−1,n−1)+f(m,n−1),0<m<n Find the value of the function.
Input
Two nonnegative integers n and m (0≤n,m≤20).
Output
Print the value of the function f(m,n).
Examples