#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/types.h>
#include <unistd.h>

int main(void)
{
    srand((unsigned)(time(NULL)+(getpid()<<16)));
    printf("%f\n", (double)rand()/RAND_MAX*2 - 1);
    return 0;
}