#include<iostream>
#include<vector>
#include<cmath>

using namespace std;

int main(){
double x;
int i=0;
double a[100];
while(cin>>x)
{
a[i]=(sqrt(x));
i++;
}

for(int j=i-1;j>=0;j--)printf("%.4f\n",a[i]);
return 0;
}

1) Change double to long double. 10^18 is much more, than double type can contain.
2) The size of your array may be greater than 100.