    01: #include <iostream>
    02: #include <string>
    03: #include <cmath>
    04:
    05: using namespace std;
    06:
    07: int main()
    08: {
    09:    double area;
    10:    cout << "Please enter the area of a square: ";
    11:    cin >> area;
    12:    if (area < 0)
    13:    {
    14:       cout << "Error: Negative area.\n";
    15:       return 1;
    16:    }
    17:
    18:    /* now we know that area is >= 0  */
    19:
    20:    double length = sqrt(area);
    21:    cout << "The side length of the square is "
    22:       << length << "\n";
    23:
    24:    return 0;
    25: }