递归求和，为什么输出总是sum=1

Question

#include
int Sum(int n);
int main()
{
int sum,n;
printf("Please input n:");
scanf("%d",&n);
if (n<=0)
{
printf("data error!\n");
}
else
{
sum=Sum(n);
printf("sum=%d\n",sum);
}
}
int Sum(int n)
{
int i;
static int a=0;
for (i=1;i<=n;i++)
{
a+=i;
return a;
}
}

Answer 1

int Sum(int n)
{
if (n == 0) return 0;
return n + Sum(n - 1);
}