to modify a file containing an integer and doubling it

#include<stdio.h>

int main()

{

    FILE *ptr;

    FILE *fptr;

    int n;

    ptr=fopen("double.txt","r");

    fscanf(ptr,"%d",&n);

fclose(ptr);

fptr=fopen("double.txt","w");

fprintf(fptr,"%d",2*n);

fclose(fptr);

}