#不知道怎么看那个WA的错误

int catalan_n(int n)
{
	if(n>0)
		return catalan_n(n-1)*(4*n-2)/(n+1);
	else
		return 1;
}
int main()
{
	int i,n,sum = 0;
	scanf("%d",&n);
	for(i = 0;i < n;i++)
		sum+= catalan_n(i)*catalan_n(n-i-1);
	printf("%d",sum);
	return 0;
}

wrong answer On line 1 column 2, read 2, expected 7