n = int(input())
m = int(input())
if n<m: print("NaN"); exit()
dp = [[0]*(n+1) for _ in range(n+1)]
for i in range(1,n+1): dp[i][1] = 1
for i in range(2,n+1):
    for j in range(2,i+1):
        dp[i][j] = j*dp[i-1][j-1]+j*dp[i-1][j]
print(dp[n][m])