#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <inttypes.h>
#include <stdint.h>
#include <fcntl.h>

int pingpong(int fdr, int fdw, int id, int max_num){
    while (1) {
        int x;
        if (read(fdr, &x, sizeof(x)) == 0) {
            return(0);
        }
        printf("%d %d\n", id, x);
        fflush(stdout);;
        ++x;
        if (x == max_num) {
            return(0);
        }
        write(fdw, &x, sizeof(x));
    }
}

int main(int argc, char **argv)
{
    int max_num = strtol(argv[1], NULL, 10);
    if (max_num <= 1) {
        _exit(0);
    }
    int fd12[2];
    int fd21[2];
    if (pipe(fd12) < 0) {
        perror("pipe");
        _exit(1);
    }
    if (pipe(fd21) < 0) {
        perror("pipe");
        _exit(1);
    }
    int pid1, pid2;
    if ((pid1 = fork()) < 0) {
            perror("fork");
            _exit(1);
        }
    if (pid1 == 0){
        close(fd12[0]);
        close(fd21[1]);
        pingpong(fd21[0], fd12[1], 1, max_num);
        close(fd21[0]);
        close(fd12[1]);
        _exit(0);
    }

    if ((pid2 = fork()) < 0) {
            perror("fork");
            _exit(1);
        }
    if (pid2 == 0){
        close(fd21[0]);
        close(fd12[1]);
        pingpong(fd12[0], fd21[1], 2, max_num);
        close(fd12[0]);
        close(fd21[1]);
        _exit(0);
    }
    int x = 1;
    write(fd21[1], &x, sizeof(x));
    close(fd12[0]);
    close(fd21[1]);
    close(fd21[0]);
    close(fd12[1]);
    wait(NULL);
    wait(NULL);
    printf("%s\n", "Done");
    fflush(stdout);
    _exit(0);
}