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: Accepted)

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

using ll = long long;

template<typename T>
T gcd(T a, T b)
{
	if (a < b)	return gcd(b, a);
	T r;
	while ((r = a % b))
	{
		a = b;
		b = r;
	}
	return b;
}

template<typename T>
T lcm(T a, T b)
{
	return a * b / gcd(a, b);
}

template<typename T>
T ext_gcd(T a, T b, T& x, T& y)
{
	x = 1;
	y = 0;
	T x0 = 0, y0 = 1;
	while (b != 0)
	{
		T q = a / b;
		a -= q * b;
		swap(a, b);
		x -= q * x0;
		swap(x, x0);
		y -= q * y0;
		swap(y, y0);
	}
	return a;
}

template<typename T>
T inv_mod(T a, T mod)
{
	T t1, t2;
	ext_gcd(a, mod, t1, t2);
	t1 %= mod;
	if (t1 < 0)		t1 += mod;
	return t1;
}

template<typename T>
T combination_large(T n, T k, T mod)
{
	if (n < k || n < 0 || k < 0)	return 0;
	vector<T> inv(n + 1, 1), fact(n + 1, 1), fact_inv(n + 1, 1);
	for (T i = 2; i <= n; ++i)
	{
		inv[i] = mod - inv[mod % i] * (mod / i) % mod;
		fact[i] = (fact[i - 1] * i) % mod;
		fact_inv[i] = fact_inv[i - 1] * inv[i] % mod;
	}
	return fact[n] * (fact_inv[k] * fact_inv[n - k] % mod) % mod;
}

template<typename T>
T combination(T n, T k, T mod = 1)
{
	vector<vector<T>> table(n + 1, vector<T>(k + 1, 1));
	for (T i = 1; i <= n; ++i)
	{
		table[i][0] = 1;
		for (T j = 1; j <= n; ++j)
		{
			table[i][j] = (table[i - 1][j - 1] + table[i - 1][j]) % mod;
		}
	}
}


int main(int, char**)
{
	int N, M;
	scanf("%d",&N);
	scanf("%d",&M);
	std::vector<pair<int, int>> p(M);
	for(int i = 0 ; i < M ; i++){
		int a, b;
		scanf("%d",&a);
		scanf("%d",&b);
		p[i] = make_pair(a, b);
	}

	sort(p.begin(), p.end(), [](const pair<int, int>& l, const pair<int, int>& r) { return l.second < r.second; });
	int x = 0;
	int count = 0;
	for (const auto& e : p)
	{
		if (e.first <= x)	continue;
		x = e.second - 1;
		++count;
	}
	cout << count << endl;

	return 0;
}