用数组求底等差数列前N项和

#include <stdio.h>

int main()
{

int n,i,sum=0;
scanf_s("%d", &n);
int a[];
for (i=1; i < n; i++)
{
    a[i - 1] = 2 * i - 1;
    sum += a[i - 1];
}
printf("%d", sum);
return 0;

}

a定大小