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 <algorithm>
#include <iostream>
using namespace std;

const long long MAXN = 1e5 + 5, MAXA = 1e6 + 6, SQRT = 447;
long long n, m, ans;
pair <long long, long long> Arr[MAXN];

bool cmp(pair <long long, long long> a, pair <long long, long long> b) {
    return a.second < b.second;
}

int main() {
    scanf("%lld%lld", &n, &m);
    for (long long i = 0; i < m; i++) {
        scanf("%lld%lld", &Arr[i].first, &Arr[i].second);
    }
    sort(Arr, Arr + m, cmp);
    long long e = -1;
    for (long long i = 0; i < m; i++)
        if (e <= Arr[i].first) {
            ans++;
            e = Arr[i].second;
        }
    printf("%lld", ans);
}