#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    system("chcp 1251");

    FILE *inputF, *outputF;
    char input[] = "C:\\Users\\admin\\Desktop\\Studlance500\\bin\\Debug\\input.txt";
    char output[] = "C:\\Users\\admin\\Desktop\\Studlance500\\bin\\Debug\\output.txt";
    char line[100];
    size_t line_len;
    int i;

    if ((inputF = fopen(input, "rb")) == NULL)
    {
        printf("Не удалось открыть файл");
        return 0;
    }
    if ((outputF = fopen(output, "wb")) == NULL)
    {
        printf("Не удалось открыть файл");
        return 0;
    }

    fgets(line, sizeof(line), inputF);
    printf("First Line: %s", line);
    line_len = strlen(line) - 1;

    printf("Input Length - %d\n\n", line_len);

    fputs(line, outputF);

    while(!feof(inputF)){
        char symbol;
        fscanf(inputF, "%c", &symbol);

        if(symbol == line[0]){
            char temp = symbol;
            int match = 1;
            for(i = 1; i < line_len - 1; i++){
                fscanf(inputF, "%c", &temp);
                if(temp != line[i]){
                    match = 0;
                    break;
                }
            }

            if(match == 1){
                fseek(inputF, -line_len + 2, SEEK_CUR);
                printf("%c", '@');
                fputc('@', outputF);
            }
            else{
                fseek(inputF, -i, SEEK_CUR);
            }

        }
        printf("%c", symbol);
        fputc(symbol, outputF);
    }

    fclose(inputF);

    fclose(outputF);
    return 0;
}