C++算法问题2023FEB

问题

Bessie is a hungry cow. Each day, for dinner, if there is a haybale in the barn, she will eat one haybale. Farmer John does not want Bessie to starve, so some days he sends a delivery of haybales, which arrive in the morning (before dinner). In particular, on day 
, Farmer John sends a delivery of 
 haybales (
, 
).

Compute the total number of haybales Bessie will eat during the first 
 days.

INPUT FORMAT (input arrives from the terminal / stdin):
The first line contains 
 and 
 (
, 
).
The next 
 lines each contain 
 and 
. It is additionally guaranteed that 
.

OUTPUT FORMAT (print output to the terminal / stdout):
Output the number of haybales that Bessie will eat during the first 
 days.
Note that the large size of integers involved in this problem may require the use of 64-bit integer data types (e.g., a "long long" in C/C++).

SAMPLE INPUT:
1 5
1 2
SAMPLE OUTPUT:
2
Two haybales arrive on the morning of day 
. Bessie eats one haybale for dinner on day 
 and another haybale on day 
. On days 
, there are no more haybales for Bessie to eat. In total, Bessie eats 
 haybales during the first 
 days.
SAMPLE INPUT:
2 5
1 2
5 10
SAMPLE OUTPUT:
3
Two haybales arrive on the morning of day 
. Bessie eats one haybale on days 
 and 
. There are no haybales for Bessie to eat on days 
 and 
. On the morning of day 
, 
 haybales arrive. Bessie eats one haybale for dinner on day 
. In total, Bessie eats 
 haybales during the first 
 days.
SAMPLE INPUT:
2 5
1 10
5 10
SAMPLE OUTPUT:
5

 haybales arrive on the morning of day 
. Bessie eats one haybale on days 
. On the morning of day 
, another 
 haybales arrive, meaning there are 
 haybales in the barn. For dinner on day 
, Bessie eats another haybale. In total, Bessie eats 
 haybales during the first 
 days.
SCORING:
Inputs 4-7: 
Inputs 8-13: No additional constraints.

请会的大蛇帮忙看看，万分感谢！

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