Help me in solving PPSC280 problem

My issue

I tried to solve this question but didn’t get right approach and solution is showing wrong answer in console , so pls tell correct solution.

My code

void userFunction() {
    FILE *file = fopen("/mnt/codechef/input.txt", "r+");   // Open the "input.txt" file for both reading and writing
    if (file == NULL) {
        perror("Error opening file");   // Print an error message if file opening fails
        return 1;   // Return an error code

    char buffer[100];   // Buffer to store each line

    // Read lines from the file, find and replace 'Codechef'
    while (fgets(buffer, sizeof(buffer), file) != NULL) {   // Read lines from the file
        char *found = strstr(buffer, "Codechef");   // Search for 'Codechef' in the line
        while (found != NULL) {   // Continue searching within the line
            fseek(file, found - buffer, SEEK_CUR);   // Move the file pointer to the start of the match
            fputs("CodeChef", file);   // Write 'CodeChef' over 'Codechef'
            fseek(file, strlen("CodeChef") - strlen("Codechef"), SEEK_CUR);
            // Move the file pointer to maintain alignment with the rest of the content
            found = strstr(found + strlen("Codechef"), "Codechef");   // Search for the next occurrence

    fclose(file);   // Close the file

    system("cat /mnt/codechef/input.txt");


Learning course: Programming using C
Problem Link: CodeChef: Practical coding for everyone