realloc () : invalid pointer error

Question;-

Today, I write a program on realloc() in visual studio code.

Input;-

#include<stdio.h>
#include<stdlib.h>
void main()
{
int n,b;
int *ptr;
int *p,*q;
int i,c=0;
printf("\nEnter the size of array = ");
scanf("%d",&n);
ptr = (int*)malloc(n*sizeof(int));
if(ptr==NULL)
{
printf("Error,its envolved");
exit(0);
}
p = ptr;
printf("\n\nEnter Element in block;-\n\n");
for(i=0;i<n;i++)
{
scanf("%d",ptr);
c = c + *ptr;
ptr++;
}
printf("\nSum = %d\n",c);
printf("\nElements in Block;-\n\n");
for(i=0;i<n;i++)
{
printf("\n%d",*p);
p++;
}
printf("\nNew size of array = ");
scanf("%d",&b);
ptr = (int*)realloc(ptr,b*sizeof(int));
if(ptr==NULL)
{
printf("Error,its envolved");
exit(0);
}
q = ptr;
printf("\n\nEnter Element in block;-\n\n");
for(i=0;i<b;i++)
{
scanf("%d",ptr);
c = c + *ptr;
ptr++;
}
printf("\nSum = %d\n",c);
printf("\nElements in Block;-\n\n");
for(i=0;i<b;i++)
{
printf("\n%d",*q);
q++;
}

}

output;-

realloc(): invalid pointer
Aborted (core dumped)

{DEAR SIR/MAM, HOW CAN I FIX THIS? }