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 <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>
#include <limits.h>
#include <cinttypes>
#include <string.h>
#include <bits/stdc++.h>

#define max(a,b) (((a) > (b)) ? (a) : (b))
#define min(a,b) (((a) < (b)) ? (a) : (b))
#define abs(a) (((a) < 0) ? -(a) : (a))

#define FOR(var, to)          for (register int var = 0; var < to; var++)
#define FROMTO(var, from, to) for (register int var = from; var <= to; var++)

typedef uint8_t u8;
typedef uint16_t u16;
typedef uint32_t u32;
typedef uint64_t u64;
typedef int8_t s8;
typedef int16_t s16;
typedef int32_t s32;
typedef int64_t s64;

using namespace std;

static inline int ri() {
	int a;
	scanf("%d", &a);
	return a;
}

static inline s64 rs64() {
	s64 a;
	scanf("%" SCNd64, &a);
	return a;
}

static inline void wi(int a) {
	printf("%d", a);
}

static inline void wu64(u64 a) {
	printf("%" PRIu64, a);
}

typedef struct {
	int a;
	int b;
}Need;
int com_need(const void *a, const void *b) {
	return (((Need*)a)->b - ((Need*)b)->b);
}

int main() {
	int n = ri();
	int m = ri();
	Need a[m];
	FOR(i,m) a[i].a=ri()-1,a[i].b=ri()-1;
	
	qsort(a,m,sizeof(Need),com_need);
	
	int res = 0;
	
	int cur_r = -1;
	FOR(i,m) {
		// printf("%d\n", a[i].b);
		if (!(a[i].a < cur_r)) {
			res++;
			cur_r = a[i].b;
		}
	}
	
	printf("%d", res);
	
	return 0;
}