#include<iostream>
#include<algorithm>
using namespace std;

int main(){
    double x;
    cin >> x;
    double l = 0, r = max(x, 1);
    while(r - l > 1e-8){//保留五位小数1e-7,否则6位小数1e-8，经验值，比有效位数多2
        double mid = (l + r) / 2;
        if(mid * mid >= x) r = mid;
        else l = mid;
    }
   printf("%lf", l);

   return 0;
}

法二：不用精度，直接迭代一百次

#include<iostream>
#include<algorithm>
using namespace std;

int main(){
    double x;
    scanf("%lf", &x);
    double l = 0, r = max(x, 1);
    for(int i = 0; i < 100; i ++ ){
        double mid = (l + r) / 2;
        if(mid * mid >= x)r = mid;
        else l = mid;
    }
    printf("%lf", l);

    return 0;
}