Program crashing while deleting an element. Other two cases are working perfectly

Question

#include<stdio.h>


int main()
{

 int i,n,roll,ch;
 char name[20];
 struct student
 {
     int roll;
     char name[20];
 }*ptr,*temp;
 printf("Enter the number of students:");
 scanf("%d",&n);
 ptr=(struct student*)malloc(n*sizeof(struct student));
 for(i=0;i<n;i++)
 {
     printf("Enter the roll number:");
     scanf("%d",&ptr[i].roll);
     printf("Enter the name:");
     scanf("%s",ptr[i].name);
 }
 while(ch!=4){
 printf("\n1.Insert\n2.Delete\n3.Display\n4.Exit");
 printf("Enter your choice:");
 scanf("%d",&ch);


 switch(ch)
 {
     case 1:
         printf("Enter the roll number:");
         scanf("%d",&roll);
         printf("Enter the name:");
         scanf("%s",name);
         temp=(struct student*)malloc(n*sizeof(struct student));
         for(i=0;i<n;i++)
         {
             temp[i].roll=ptr[i].roll;
             strcpy(temp[i].name,ptr[i].name);
         }
         temp[n].roll=roll;
         strcpy(temp[n].name,name);
         free(ptr);
         ptr=temp;
         n++;
         for(i=0;i<n;i++)
         {
             printf("%d %s\n",ptr[i].roll,ptr[i].name);
         }
         break;
     case 2:
         printf("Enter the roll number:");
         scanf("%d",&roll);
         temp=(struct student*)malloc(n*sizeof(struct student));
         for(i=0;i<n;i++)
         {
             if(ptr[i].roll==roll)
             {
                 i++;
             }
             temp[i-1].roll=ptr[i].roll;
             strcpy(temp[i-1].name,ptr[i].name);
         }
         free(ptr);
         ptr=temp;
         n--;
         for(i=0;i<n;i++)
         {
             printf("%d %s\n",ptr[i].roll,ptr[i].name);
         }
         break;
     case 3:
         for(i=0;i<n;i++)
         {
             printf("%d %s\n",ptr[i].roll,ptr[i].name);
         }
         break;
     }
 }

 return 0;

}

The error message which I am getting-

Enter the number of students:2
Enter the roll number:87
Enter the name:Arya
Enter the roll number:45
Enter the name:Yo

1.Insert
2.Delete
3.Display
4.ExitEnter your choice:3
87 Arya
45 Yo

1.Insert
2.Delete
3.Display
4.ExitEnter your choice:1
Enter the roll number:88
Enter the name:Nair
87 Arya
45 Yo
88 Nair

1.Insert
2.Delete
3.Display
4.ExitEnter your choice:2
Enter the roll number:88

Process returned -1073741819 (0xC0000005)   execution time : 45.341 s
Press any key to continue.

What am I doing wrong in the second case? Earlier I was trying to write the code by using double pointers but I was not able to implement it.

in case 2 my logic was to create a new temporary array and store all the elements other than the one with that "roll". i reviewed the code again and again and even asked my peers