Twin Beacon Frequency Pairs

A coastal traffic-control authority has deployed n signal beacons along a shipping lane. Each beacon continuously broadcasts on two channels, transmitting one frequency value on channel one and another on channel two (a beacon may even transmit the same value on both channels). Two beacons are called a twin pair if the pair of frequencies they broadcast matches exactly, regardless of which channel carried which value — that is, {channel-one value, channel-two value} is the same unordered pair for both beacons. Count how many twin pairs exist among all n beacons.

Input format

The first line contains a single integer n, the number of beacons. Each of the next n lines contains two integers a and b — the frequencies broadcast by that beacon on channel one and channel two, respectively.

Output format

Print a single integer: the number of index pairs (i, j) with i < j such that beacon i and beacon j form a twin pair.

Constraints

• 1 <= n <= 40000
• 1 <= a, b <= 9

6
1 2
2 1
3 4
5 6
6 5
3 4

3

5
1 2
1 2
1 1
1 2
2 2

3