#include <stdlib.h> /* atoi */
#include <stdio.h> /* printf, scanf */

int main(int argc, char* argv[])
{
    int len, i, *buf, acc;
    if (argc != 2) {
        printf("usage: <array length>\n");
        return 1;
    }
    len = atoi(argv[1]);
    if (len == 0 || len % 2 == 0) {
        printf("bad length\n");
        return 1;
    }
    buf = malloc(len * sizeof(int));
    for (i = 0; i < len; i++) {
        printf("%d> ", i + 1);
        scanf("%d", &buf[i]);
    }
    acc = 0;
    for (i = 0; i < len; i++) {
        acc ^= buf[i];
    }
    printf("result: %d\n", acc);
    return 0;
}