The editing of the C binary file is looped

You must replace every third number in the binary file with the same number with the opposite sign. The numbers are represented by the double type. When you try to overwrite a part of the file, it loops.

#include <stdio.h>
#include <stdlib.h>
#define MAX_PATH 261

int main (int argc, char *argv[])
{
    FILE *input_file;
    char *filename;
    int n = 0;
    double number;
    filename = (char*) malloc (MAX_PATH*sizeof(char));
    if (argc != 2)
    {
        printf ("Enter filename: ");
        scanf ("%s", filename);
    }   else filename = argv[1];
    if ((input_file = fopen (filename, "r+b")) == NULL)
    {
        perror ("Cannot open input file");
        return 1;
    }
    while (!feof(input_file))
    {
        n += fread (&number, sizeof(double), 1, input_file);
        if (n % 3 == 0 && n != 0)
        {
            number *= -1;
            fseek (input_file, (n-1)*sizeof(double), SEEK_SET);
            fwrite (&number, sizeof(double), 1, input_file);
        }
    }
    fclose (input_file);
    free (filename);
    system ("pause");
    return 0;
}