BZOJ2155(?) R集合 （卡特兰数）

Orz Freopen大佬

CODE

fac = [0]*1005
def C(n, m):
    return fac[n] // fac[m] // fac[n-m]
n = int(input())
fac[0] = 1
for i in range(1, n+1):
    fac[i] = fac[i-1] * i
ans = 0
for i in range(2, n//2+1):
    ans += C(n, 2*i) * C(2*i-1, i+1)
print(ans)