Mirko's village has only one long street stretching from east to west with \(M\) houses. Each house has a unique house number, starting with \(1\) and ending with \(M\).

Recent storm took out most phone lines so the mayor financed construction of a new one. Mirko is interested in the popularity of this new phone network, so he infiltrated its construction and placed special detectors on some points.

Detector detects any phone call made between two houses, as long as one of them is eastward and the other westward from the point the detector is installed.

At the end of the first month, Mirko removed all detectors and now wonders what is the smallest number of phone calls that could have been made during that month.

The first line of input contains two integers \(N\) (\(1 \le N \le 100\,000\)), number of detectors, and \(M\) (\(N < M \le 1\,000\,000\,000\)), number of houses in the village.

Next \(N\) lines contain two numbers each: \(P_i\) (\(1 \le P_i < M\)), and \(C_i\) (\(1 \le C_i \le 1\,000\,000\,000\)), the position and total number of phone calls detected by detector numbered \(i\). We say that a detector is on position \(P_i\) if and only if it is between houses numbered \(P_i\) and \(P_i+1\).

There will never be more than one detector on the same position.

Output a single integer, the minimal number of phone calls made.

Scoring: In test cases worth \(50\%\) of total points, \(N\) and \(C\) will be smaller than \(1\,000\).

riseoj 작성

출처 COCI 2009/2010 Contest 3