#include<stdio.h>
#include<math.h>
unsigned long long int a,i,s[10000000];
int main()
{
    double b;
    i=0;
    //freopen("input.txt","r",stdin);
    while(scanf("%llu",&a)!=EOF)
    {
        s[i]=a;
        i++;


    }
    while(i>0)
    {
        i--;
        b=sqrt(s[i]);
        printf("%.4lf\n",b);
    }

    return 0;
}

Plz use "%.4f" instead "%.4lf" to printf. :)

why %.4lf is not acceptable?

you are fool?  this is no need reason ,this is fixed 格式

To use sqrt() from math.h you need to add -lf flag while compiling, I believe.