#include <iostream>
#include <cmath>
#include <vector>
using namespace std;

int main()
{
    cout.setf(cout.fixed);
    cout.setf(cout.showpoint);
    cout.precision(4);
    vector <double> a;
    double c;
    while(cin>>c)
    {
        a.push_back(sqrt(c));
    }
    for(int i = a.size()-1;i>=0;i--)
        cout << a[i] << endl;
}

You should use long long instead of double

Probably gcc?
Try to select MSVC compiler. Also you can try using printf/scanf instead of cin/cout.

1) How?
2) Why?