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: Runtime Error)

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

#define REP(i, x) for (int i = 0, i##_len = (int)(x); i < i##_len; i++)
#define REP1(i, x) for (int i = 1, i##_len = (int)(x); i <= i##_len; i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREP1(i, x) for (int i = ((int)(x)); i > 0; i--)
#define ALL(v) (v).begin(), (v).end()
#define debug(x) cerr << #x << ": " << x << '\n'
#define pb(x) push_back(x)
#define mp(x, y) make_pair(x, y)
const int MOD = (int)1e9 + 7;
const int INF = 1145141919;
using ll = long long;
using ull = unsigned long long;
using i_i = pair<int, int>;
using l_l = pair<ll, ll>;
using d_d = pair<double, double>;
using vint = vector<int>;
using vll = vector<ll>;
using vbool = vector<bool>;

int main() {
    int N, M;
    cin >> N >> M;
    vint imosp(10005);
    vint imosn(10005);
    REP(i, M) {
        int a, b;
        cin >> a >> b;
        imosp[a]++, imosn[b]++;
    }
    int ans = 0;
    int now = 0;
    REP(i, N) {
        if (imosn[i] && now) {
            ans++;
            now = 0;
        }
        now += imosp[i];
    }
    if (now)
        ans++;
    cout << ans;
}