#define _POSIX_C_SOURCE 2
#include <ctype.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>

int read_line(int fd, char *buf, size_t len, int *bye) {
    char c;
    ssize_t st;
    if (len == 0) return 0; // precaution
    while ((st = read(fd, &c, 1))) {
        if (st < 0) return errno;
        if (c == '\n') break;
        else if (len <= 1) return -1; // buffer too small
        else {
            *buf = c;
            buf++;
            len--;
        }
    }
    *buf = '\0';
    if (st == 0) *bye = 1;
    return 0;
}

int line_is_empty(char *data) {
    for(;*data != '\0'; data++) {
        if (!isspace(*data)) return 0;
    }
    return 1;
}

int parse_line(char *data, int *result) {
    if (data == NULL || result == NULL) return 0;
    char *tok;
    int howmany;
    for (howmany = 0;;howmany++) {
        if (!howmany) tok = strtok(data, " ");
        else tok = strtok(NULL, " ");
        if (tok == NULL) break;
        if (howmany > 1) return -1; // too many fields
        errno = 0;
        *result = (int) strtol(tok, NULL, 10);
        result++; // must be valid
        if (errno) return errno;
    }
    if (howmany != 2) return -2; // too few fields
    return 0;
}

int sign(long num) {
    if (num > 0) return 1;
    else if (num == 0) return 0;
    else return -1;
}

int cmp(const void *one, const void *two) {
    return sign(*((int *) one) - *((int *) two));
}

void complain(int msg, int where) {
    printf("On line %d: ", where);
    if (msg > 0) puts(strerror(msg));
    else printf("internal error: %d", msg);
}

int main() {
    char buf[1024];
    int line;
    int bye = 0;
    int i[2];
    int id_1[1024];
    int id_2[1024];
    int st;
    for(line = 0; line < 1024; line++) {
        if ((st = read_line(0, buf, 1024, &bye))) {
            complain(st, line + 1);
            return 1;
        }
        else if (line_is_empty(buf) && !bye) {
            line--; // 200iq
            continue;
        }
        if (bye) break;
        if ((st = parse_line(buf, i))) {
            complain(st, line + 1);
            return 1;
        }
        id_1[line] = i[0];
        id_2[line] = i[1];
    }
    qsort(id_1, line, sizeof(int), cmp);
    qsort(id_2, line, sizeof(int), cmp);
    int sum = 0;
    for (int iter = 0; iter < line; iter++) {
        sum += id_2[iter] - id_1[iter];
    }
    printf("Total distance is %d\n", sum);
}