Can`t understand what is the problem

#include <stdio.h>
#include <math.h>

int main()
{
    unsigned long long int a[4096], b;
    int counter=0;
    while(scanf("%llu", &b))
    {
        a[counter]=b;
        counter++;
    }

    for(int i=counter-1; i>=0; i--)
        printf("%.15Lg\n", (long double)sqrt(a[i]));
    return 0;
}

1) while(scanf("%llu", &b) != EOF)
2) Maybe it's ok, but why do you printf("%.15Lg\n)? 4 symbols after the dot is not ennough, or I don't understand sth?