//
//  main.cpp
//  curcubeu
//
//  Created by Eusebiu Rares on 21/09/2020.
//  Copyright © 2020 Eusebiu Rares. All rights reserved.
//

#include <iostream>
#include "fstream"
#include "algorithm"

template <typename T>
class InputReader {
private:
	FILE *input_file ;
	static const int SIZE = (1 << 17) ;
	int point ;
	char buffer[SIZE] ;
	__attribute__ ( (always_inline)) void advance() {
		++ point ;
		if (point == SIZE) {
			point = 0 ;
			fread (buffer, SIZE, 1, input_file) ;
		}
	}
public:
	InputReader() {}
	InputReader (const char *file_name) {
		input_file = fopen (file_name, "r") ;
		point = 0 ;
		fread (buffer, SIZE, 1, input_file) ;
	}
	__attribute__ ( (always_inline)) InputReader &operator >> (T &n) {
		for (; !isdigit (buffer[point]) ; advance()) ;
		n = 0 ;
		for (; isdigit (buffer[point]) ; advance()) {
			n = n * ( (1 << 3) + (1 << 1)) + buffer[point] - '0' ;
		}
		return *this ;
	}
} ;

InputReader<int> input("curcubeu.in") ;
std::fstream output ("curcubeu.out",  std::ios::out) ;

struct DC {
    int c, poz ;
} ;
 
const int MV = 2005 ;
 
int n, job, dp[2][MV] ;
DC act[MV] ;
 
int main() {
	int i, j, mx=-(1<<30), s ;
	input >> n >> job ;
	for (i = 1 ; i <= n ; ++ i) {
		input >> act[i].poz >> act[i].c ;
	}
	std::sort(act + 1, act + n + 1,  [](DC A, DC B) {
			return A.poz < B.poz ;
	}) ;
	for (i = 1 ; i <= n ; ++ i) {
		dp[0][i] = act[i].c - job ;
		for (j = i - 1, s = act[i].c ; j >= 1 ; -- j) {
			if (act[j].c >= act[i].c)
				s += act[i].c ;
			dp[0][i] = std::max(dp[0][i], s - (act[i].poz - act[j].poz + 1) * job) ;
		}
		for (j = i + 1, s = 0 ; j <= n ; ++ j) {
			if (act[j].c >= act[i].c)
				s += act[i].c ;
				dp[1][i] = std::max(dp[1][i], s - (act[j].poz - act[i].poz) * job) ;
			}
		mx = std::max(mx, dp[0][i] + dp[1][i]) ;
	}
	output << mx ;
}