P3021 [USACO11MAR] Bovine Bridge Battle S
Background
给定N（4
Description
Each of Farmer John's N (4
Input Format
\* Line 1: A single integer: N
\* Lines 2..N+1: Line i+1 contains two space-separated integers: X\_i and Y\_i
Output Format
\* Line 1: A single integer that is the number of sets of 4 cows that form valid groups for bridge.