求大家帮我看看这个程序哪里出问题了，只能输入几个。。
#include <stdio.h>
#define SIZE 5

struct student //定义结构体
{
    int ID; //学号
    char name[20]; //姓名
    int math_marks; //数学成绩
    int computer_marks; //计算机成绩
    int english_marks; //英语成绩
    int average;
};
void ShellSort(struct student a[],int N)//希尔排序
{
    struct student t;
     int i,j,gap;
     for (gap=N/2;gap>0;gap=gap/2)
      {
         for(i=gap;i<=N;i++)
//           for(i=gap+1;i<=N;i++)
        {              
             j=i-gap;
              while(j>=0)
              {
                if(a[j].average>a[j+gap].average)
                {
                 t=a[j];
                 a[j]=a[j+gap];
                 a[j+gap]=t;
                 j=j-gap;
                }
                 else break;
               }
        }
        }
}
int main()
{
    struct student a[SIZE];
  
 // int a[] = {45,65,11,21,36,7,98,100,73,61};
  
   int i;
   for(i=0;i<SIZE;i++)//输入学生信息
    {
        printf("请输入第%d个学生的学号",i+1);
        scanf("%d",&a[i].ID);
        printf("请输入第%d个学生的姓名",i+1);
        scanf("%s",&a[i].name);
        printf("请输入第%d个学生的数学成绩",i+1);
        scanf("%d",&a[i].math_marks);
        printf("请输入第%d个学生的计算机成绩",i+1);
        scanf("%d",a[i].computer_marks);
        printf("请输入第%d个学生的英语成绩",i+1);
        scanf("%d",&a[i].english_marks);
        printf("请输入第%d个学生的平均分",i+1);
        scanf("%d",&a[i].average);
        
    }
  ShellSort(a,SIZE);
  for(i=0;i<SIZE;i++)
  {
     printf("%d,%s,%d,%d,%d,%d",a[i].ID,a[i].name,a[i].math_marks,a[i].computer_marks,a[i].english_marks,a[i].average);
  }
   return 0;           
}