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 Python (3.4.3) (judge verdict: Accepted)

n, m = map(int, input().split())
ab = [list(map(int, input().split())) for _ in range(m)]

ends = [[[], []] for _ in range(n+1)]
for i, (a, b) in enumerate(ab, start = 1):
    ends[a][0].append(i)
    ends[b][1].append(i)
# for e in ends: print(e)

ans = 0
orders = set([])
for start, end in ends:
    if len(end) > 0:
        rm_bridge = False
        for e in end:
            if e in orders:
                orders.remove(e)
                rm_bridge = True
        if rm_bridge:
            orders = set([])
        ans += int(rm_bridge)
    if len(start) > 0:
        for s in start:
            orders.add(s)
    # print(orders)
print(ans)