Problem:
Score : 400 points

Problem Statement

There are N islands lining up from west to east, connected by N-1 bridges.

The i-th bridge connects the i-th island from the west and the (i+1)-th island from the west.

One day, disputes took place between some islands, and there were M requests from the inhabitants of the islands:

Request i: A dispute took place between the a_i-th island from the west and the b_i-th island from the west. Please make traveling between these islands with bridges impossible.

You decided to remove some bridges to meet all these M requests.

Find the minimum number of bridges that must be removed.

Constraints

All values in input are integers.

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq a_i < b_i \leq N

All pairs (a_i, b_i) are distinct.

Input

Input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M

Output

Print the minimum number of bridges that must be removed.

Sample Input 1

5 2
1 4
2 5

Sample Output 1

1

The requests can be met by removing the bridge connecting the second and third islands from the west.

Sample Input 2

9 5
1 8
2 7
3 5
4 6
7 9

Sample Output 2

2

Sample Input 3

5 10
1 2
1 3
1 4
1 5
2 3
2 4
2 5
3 4
3 5
4 5

Sample Output 3

4

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;

const int maxn = 1e5 + 10;
int n, m, ans, c[maxn];

struct dispute {
	int l, r;
	bool operator < (const dispute& d) const {
		return d.r < r;
	}
} a[maxn];

void add(int pos) {
	for (; pos <= n; pos += pos & -pos) {
		c[pos]++;
	}
}

int query(int pos) {
	int res = 0;
	for (; pos; pos -= pos & -pos) {
		res += c[pos];
	}
	return res;
}

int main() {
	scanf("%d %d", &n, &m);
	for (int i = 1; i <= m; i++) {
		scanf("%d %d", &a[i].l, &a[i].r);
	}
	sort(a + 1, a + m + 1);
	for (int i = 1; i <= m; i++) {
		if (!(query(a[i].r) - query(a[i].l))) {
			add(a[i].l), ans++;
		}
	}
	printf("%d", ans);
	return 0;
}