时间限制：C/C++/Rust/Pascal 1秒，其他语言2秒
空间限制：C/C++/Rust/Pascal 32 M，其他语言64 M
64bit IO Format: %lld
空间限制：C/C++/Rust/Pascal 32 M，其他语言64 M
64bit IO Format: %lld
题目描述
The cows are so modest they want Farmer John to install covers around the circular corral where they occasionally gather. The corral has circumference C (1 <= C <= 1,000,000,000), and FJ can choose from a set of M (1 <= M <= 100,000) covers that have fixed starting points and sizes. At least one set of covers can surround the entire corral.
Cover i can be installed at integer location x_i (distance from starting point around corral) (0 <=
< C) and has integer length l_i (1 <= 
<= C).
Cover i can be installed at integer location x_i (distance from starting point around corral) (0 <=
FJ wants to minimize the number of segments he must install. What is the minimum number of segments required to cover the entire circumference of the corral?
Consider a corral of circumference 5, shown below as a pair of connected line segments where both '0's are the same point on the corral (as are both 1's, 2's, and 3's).
Three potential covering segments are available for installation:
Start Length
i x_i l_i
1 0 1
2 1 2
3 3 3
0 1 2 3 4 0 1 2 3 ...
corral: +---+---+---+---+--:+---+---+---+- ...
11111 1111
22222222 22222222
333333333333
|..................|
As shown, installing segments 2 and 3 cover an extent of (at least) five units around the circumference. FJ has no trouble with the overlap, so don't worry about that. 输入描述:
* Line 1: Two space-separated integers: C and M
* Lines 2..M+1: Line i+1 contains two space-separated integers:
输出描述:
* Line 1: A single integer that is the minimum number of segments required to cover all segments of the circumference of the corral