After making bad dives into swimming pools, Wilbur wants to build a swimming pool in the shape of a rectangle in his backyard. He has set up coordinate axes, and he wants the sides of the rectangle to be parallel to them. Of course, the area of the rectangle must be positive. Wilbur had all four vertices of the planned pool written on a paper, until his friend came along and erased some of the vertices.

Now Wilbur is wondering, if the remaining n vertices of the initial rectangle give enough information to restore the area of the planned swimming pool.

The first line of the input contains a single integer n (1 ≤ n ≤ 4) — the number of vertices that were not erased by Wilbur's friend.

Each of the following n lines contains two integers xi and yi ( - 1000 ≤ xi, yi ≤ 1000) —the coordinates of the i-th vertex that remains. Vertices are given in an arbitrary order.

It's guaranteed that these points are distinct vertices of some rectangle, that has positive area and which sides are parallel to the coordinate axes.

Print the area of the initial rectangle if it could be uniquely determined by the points remaining. Otherwise, print  - 1.

In the first sample, two opposite corners of the initial rectangle are given, and that gives enough information to say that the rectangle is actually a unit square.

In the second sample there is only one vertex left and this is definitely not enough to uniquely define the area.

题意:输入若干个点判断能否确定一个矩形的面积，矩形平行于坐标轴

开始正常写，提交WA,然后改，始终没能找到错误。最后才发现自己思维太不严谨了

 #include <iostream>

 #include <cstdio>

 #include <algorithm>

 #include <cstring>

 #include <cmath>

 using namespace std;

 int flag,n;

 struct node

 {

     int x,y;

 };

 node a[];

 int main()

 {

     while(scanf("%d", &n) != EOF)

     {

         flag = ;

         int s;

         for(int i = ; i < n; i++)

             scanf("%d%d", &a[i].x, &a[i].y);

         if(n < )

             flag = ;

         else

         {

             if(n == )

             {

                 if(a[].x == a[].x || a[].y == a[].y)

                 {

                     flag = ;

                 }

                 else

                 {

                     s = abs(a[].x - a[].x) * abs(a[].y - a[].y);

                 }

             }

             else

             {

                 if ((a[].x == a[].x || a[].y == a[].y) && (a[].x == a[].x || a[].y == a[].y) ) //就是落了这个判断条件。。。。。。

                     s = abs(a[].x - a[].x) * abs(a[].y - a[].y);

                 else

                 for(int i = ; i < n; i++)

                 {

                     if(a[].x == a[i].x || a[].y == a[i].y)

                         continue;

                     else

                         s = abs(a[].x - a[i].x) * abs(a[].y - a[i].y);

                 }

             }

         }

         if(flag)

             printf("-1\n");

         else

             printf("%d\n",s);

     }

     return ;

 }