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)

// #pragma GCC optimize(3, "Ofast", "inline")
#include <bits/stdc++.h>
#define String std::string
#define fi first
#define se second
#define mp std::make_pair
#define Pair std::pair<int, int>
#define Map std::map< int, int >
#define Vector std::vector<int>
 
using std::cin;
using std::cout;
using std::endl;
 
typedef long long ll;
typedef unsigned long long ull;
 
const int N = 100000 + 5;
const int M = 1000 + 5;
const int K = 1e7 + 5;
const int MOD = 1e9 + 7;
const int inv2 = 500000004;
const int LOG = 40;
const ull BASE = 19260817;
const int dx[] = { 0, 1, -1, 0, 1, -1, 1, -1 };
const int dy[] = { 1, 0, 0, -1, 1, -1, -1, 1 };
const double PI = acos(-1);

template<typename T> inline bool chkmin(T &a, const T &b) { return a > b ? a = b, 1 : 0; }
template<typename T> inline bool chkmax(T &a, const T &b) { return a < b ? a = b, 1 : 0; }
 
template<class T>
inline T readT() {
    T res = 0, f = 1;
    char ch = getchar();
    for (; !isdigit(ch); ch = getchar()) if (ch == '-') f = -1;
    for (; isdigit(ch); ch = getchar()) res = (res << 1) + (res << 3) + ch - '0';
    return res * f;
}
#define read readT<int>

int n, m, ans;
Pair br[N];
std::vector< Pair > v[N];
std::stack<int> s;
bool vis[N], ins[N];

int main() {
    cin >> n >> m;
    for (int i = 1; i <= m; i++) {
        scanf("%d %d", &br[i].fi, &br[i].se);
        v[br[i].fi].push_back(mp(br[i].fi, -1));
        v[br[i].se].push_back(mp(br[i].se, i));
    }
    for (int i = 1; i <= n; i++) {
        for (int j = 0; j < v[i].size(); j++) {
            if (v[i][j].se == -1) {
                if (!ins[i]) s.push(i);
            } else {
                if (vis[br[v[i][j].se].fi]) continue;
                else {
                    ans++;
                    while (!s.empty()) {
                        vis[s.top()] = true;
                        s.pop();
                    }
                }
            }
        }
    }
    printf("%d\n", ans);
    return 0;
}