CF24E.Berland collider

Recently the construction of Berland collider has been completed. Collider can be represented as a long narrow tunnel that contains n particles. We associate with collider 1-dimensional coordinate system, going from left to right. For each particle we know its coordinate and velocity at the moment of start of the collider. The velocities of the particles don't change after the launch of the collider. Berland scientists think that the big bang will happen at the first collision of particles, whose velocities differs in directions. Help them to determine how much time elapses after the launch of the collider before the big bang happens.

最近，贝尔兰对撞机的建造已经完成。该对撞机可被建模为一条细长的狭长隧道，其中包含 $n$ 个粒子。我们为该对撞机建立一个从左至右的一维坐标系。对于每个粒子，我们已知其在对撞机启动时刻的位置坐标和速度。对撞机启动后，各粒子的速度保持不变。贝尔兰科学家认为，当两个运动方向相反的粒子首次发生碰撞时，将引发“大爆炸”。请帮助他们确定：对撞机启动后，经过多长时间将发生“大爆炸”。

The first line contains single integer n (1 ≤ n ≤ 5·105) — amount of particles in the collider. Next n lines contain description of particles. Each particle is described by two integers x__i, v__i ( - 109 ≤ x__i, v__i ≤ 109, v__i ≠ 0) — coordinate and velocity respectively. All the coordinates are distinct. The particles are listed in order of increasing of coordinates. All the coordinates are in meters, and all the velocities — in meters per second. The negative velocity means that after the start of collider the particle will move to the left, and the positive — that the particle will move to the right.

第一行包含一个整数 $n$（$1 \leq n \leq 5 \cdot 10^5$）—— 粒子对撞机中粒子的数量。接下来的 $n$ 行描述了各个粒子。每个粒子由两个整数 $x_i$、$v_i$（$-10^9 \leq x_i, v_i \leq 10^9$，且 $v_i \neq 0$）描述，分别表示其初始坐标（单位：米）和速度（单位：米/秒）。所有坐标的值互不相同，且粒子按坐标升序排列。负速度表示粒子在对撞机启动后将向左运动，正速度表示粒子将向右运动。

If there will be no big bang, output -1. Otherwise output one number — how much time in seconds elapses after the launch of the collider before the big bang happens. Your answer must have a relative or absolute error less than 10 - 9.

如果不会发生大爆炸，则输出 $-1$。否则输出一个数字——从粒子对撞机启动到发生大爆炸之间经过的时间（单位：秒）。你的答案必须满足相对误差或绝对误差小于 $10^{-9}$。