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: Wrong Answer)

N, M = map(int, input().split())
ABs = sorted([list(map(int, input().split())) for _ in range(M)])

groups = [(ABs[0][0], ABs[0][1])]

gidx = 0
flag = True
for e in ABs[1:]:
    for gidx, g in enumerate(groups):
        if (g[1] < e[0]) or (e[1] < g[0]):
            groups.append((e[0], e[1]))
            break
        elif (g[0] <= e[0] or e[1] <= g[1]):
            groups[gidx] = (max(g[0], e[0]), min(g[1], e[1]))
            break

print(len(groups))