merge sort program in C by codepiova programmer

#include<stdio.h>
int merge(int *,int ,int ,int);
void mergesort(int *,int ,int);
main()
{
    int n,a[40],i,p,r;
    printf("Enter total no of element in an array\n");
    scanf("%d",&n);
     printf("Enter total %d element in an array\n",n);
    for(i=1;i<=n;i++)
        scanf("%d",&a[i]);
    printf("\nElement Before sorting\n");
    for(i=1;i<=n;i++)
        printf("%d\t",a[i]);
        p=1;
        mergesort(a,p,n);
         printf("\nElement After sorting\n");
    for(i=1;i<=n;i++)
        printf("%d\t",a[i]);
}
void mergesort(int *a,int p,int r)
{
    int q;
    if(p<r)
    {
    q=(p+r)/2;
    mergesort(a,p,q);
    mergesort(a,q+1,r);
    merge(a,p,q,r);
    }
}
int merge(int *a,int p,int q,int r)
{
    int n1,n2,l[30],m[30],i,j,k;
    n1=q-p+1;
    n2=r-q;
    for(i=1;i<=n1;i++)
        l[i]=a[p+i-1];
    for(j=1;j<=n2;j++)
        m[j]=a[q+j];
    l[n1+1]=1234567897;
    m[n2+1]=1234567898;
    i=1;j=1;
    for(k=p;k<=r;k++)
    {
        if(l[i]<=m[j])
        {
            a[k]=l[i];
            i=i+1;
        }
        else
        {
            a[k]=m[j];
            j=j+1;
        }
    }
    return a;
}