Codeforces Round #320 (Div. 2) C. A Problem about Polyline ( 数学 )

题目链接

解题报告：
考虑一个周期的话，若（a，b）在其上必然满足 x >= b ，a>=b 。
很明显折线会经过 (a-b,0)（a+b,0）两点，令 c=a±b，y=c/(2*x);a
x=c/(2*y)>=b ==>> y<=c/(2*b) ==>> y_max=c/(2*b)向下取整
最后去min。

#define first f
#define second s
#define ll long long
#define mp make_pair
#define pb push_back
#define pf push_front
#define lb lower_bound
#define ub upper_bound
#define pii pair<int,int>
#include <bits/stdc++.h>
#define mem(a,b) memset(a,b,sizeof(a))
using namespace std;
const int MOD=1e9+7;
const int maxn=1e5+5;
const double eps=1e-9;
const int inf=0x3f3f3f3f;
const double PI=acos(-1.0);
const double e=2.718281828459;


int main()
{
    int x,y;
    scanf("%d%d",&x,&y);
    if(y>x){
        printf("-1\n");
    }
    else{
           int mi=x+y,mx=x-y;
            printf("%.12f\n",min(mi*0.5/(mi/(2*y)),mx*0.5/(mx/(2*y))));
    }
    return 0;
}