I have written this Merge Sort program in C. It shows "Segmentation fault (core dumped)" after implementation of arrays

#include<stdio.h>
void mergesort();
void merge();
int main()
{
 int a[40],n;
 printf("\nEnter the number of elements:");
 scanf("%d",&n);
 printf("\nEnter the %d elements:",n);
 for(int i=0;i<n;i++)
 {
  scanf("%d",&a[i]);
 }
 mergesort(a,0,n-1); 
 printf("\nThe Sorted array is: ");
 for(int i=0;i<n;i++)
 {
  printf("%d ",a[i]);
 }
 return 0;
} 

void mergesort(int a[],int first,int last)
{
 int mid;
 if(first<last)
 {
  mid=(mid+last)/2;
  mergesort(a,first,mid);
  mergesort(a,mid+1,last);
  merge(a,first,mid,last);
 }
}

void merge(int a[],int first,int mid, int last)
{
 int b[50];
 int i,j,k;
 i=first;
 j=mid+1;
 k=first;
 while( i<=mid && j<=last) 
 {
  if(a[i]<=a[j])
    b[k++]=a[i++];
  else 
    b[k++]=a[j++];
 }
 if(i>mid)
 {
  while(j<=last)
   b[k++]=a[j++];
 }
 else
 {
  while(i<=mid)
   b[k++]=a[i++];
 }
  for(i=first;i<=last;i++)
  a[i]=b[i];
}

I have written this Merge Sort program in C. It shows "Segmentation fault (core dumped)" after implementation of arrays.
I don’t know what is the issue. So please help me out i have exam on the next day.
I get output like this

Enter the number of elements:5

Enter the 5 elements:1
2
3
5
4
Segmentation fault (core dumped)

>Solution :

The program has undefined behavior since you read mid when it’s uninitialized:

void mergesort(int a[], int first, int last) {
    int mid;
    if (first < last) {
        mid = (mid + last) / 2;
//             ^^^

You probably want:

        mid = (first + last) / 2;