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# 计蒜客

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There's a beach in the first quadrant. And from time to time, there are sea waves. A wave ( \$x\$ , \$y\$ ) means the wave is a rectangle whose vertexes are ( \$0\$ , \$0\$ ), ( \$x\$ , \$0\$ ), ( \$0\$ , \$y\$ ), ( \$x\$ , \$y\$ ). Every time the wave will wash out the trace of former wave in its range and remain its own trace of ( \$x\$ , \$0\$ ) -> ( \$x\$ , \$y\$ ) and ( \$0\$ , \$y\$ ) -> ( \$x\$ , \$y\$ ). Now the toad on the coast wants to know the total length of trace on the coast after n waves. It's guaranteed that a wave will not cover the other completely.

### Input

The first line is the number of waves \$n(n \le 50000)\$.

The next \$n\$ lines，each contains two numbers \$x\$ \$y\$ ,( \$0 < x\$ , \$y \le 10000000\$ )，the \$i\$-th line means the \$i\$-th second there comes a wave of ( \$x\$ , \$y\$ ), it's guaranteed that when \$1 \le i\$ , \$j \le n\$ ，\$x_i \le x_j\$ and \$y_i \le y_j\$ don't set up at the same time.

### Output

An Integer stands for the answer.

### Hint：

As for the sample input, the answer is \$3+3+1+1+1+1=10\$

```3
1 4
4 1
3 3```

`10`

• main.c