1 def func():
2 L, M = map(int, input().strip().split())
3 left, right = [], {}
4 for _ in range(M):
5 temp1, temp2 = map(int, input().strip().split())
6 left.append(temp1)
7 right[temp1] = temp2
8
9 left.sort()
10 cur_index = left[0]
11 left_index = 1
12 stack = [right[left[0]]]
13 trees = left[0]
14 while left_index < M:
15 if not stack:
16 trees += left[left_index] - cur_index
17 stack.append(right[left[left_index]])
18 left_index += 1
19 elif left[left_index] < stack[-1]:
20 stack.append(right[left[left_index]])
21 cur_index = left[left_index]
22 left_index += 1
23 elif left[left_index] >= stack[-1]:
24 trees += left[left_index] - stack[-1] - 1
25 while stack:
26 if left[left_index] > stack[-1]:
27 stack.pop(-1)
28 else:
29 break
30 stack.append(right[left[left_index]])
31 left_index += 1
32 if stack:
33 trees += L - stack[-1]
34
35 print(trees)
36
37
38 if __name__ == "__main__":
39 func()
