Файлы. Сумма элементов главной диагонали матрицы

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#define BUF 11
int main() {
    char inp[] = "matrix.txt", exp[] = "result.txt", temp[BUF];
    int summa = 0, element, not_found, not_create, not_established, is_corrupted;
    size_t size = 0, n, m;
    FILE * in, * out;
    errno_t err;
    not_found = 0;
    err = fopen_s(&in, inp, "r");
    if (err != 0) {
        printf("The file is not found!");
        ++not_found;
    } else {
        not_create = 0;
        err = fopen_s(&out, exp, "w");
        if (err != 0) {
            printf("The file is not created!");
            ++not_create;
        } else {
            not_established = 0;
            fscanf_s(in, "%s", temp, BUF);
            if (!(size = atoi(temp))) {
                printf("The size is not established!");
                ++not_established;
            } else {
                is_corrupted = 0;
                for (n = 0; n < size; n++) {
                    for (m = 0; m < size; m++) {
                        fscanf_s(in, "%s", temp, BUF);
                        if (n == m) {
                            if (!(element = atoi(temp))) {
                                printf("The file is corrupted!");
                                ++is_corrupted;
                                break;
                            } summa += element;
                        }
                    } if (is_corrupted) break;
                }
                if (!is_corrupted) {
                    _itoa_s(summa, temp, BUF, 10);
                    fputs(temp, out);
                }
            } fclose(out);
        } fclose(in);
    }
    if (not_found || not_create || not_established || is_corrupted) _getch();
    return 0;
}