#include <bits/stdc++.h>

using namespace std;

class InParser {
private:
	FILE *fin;
	char *buff;
	int sp;

	char read_ch() {
		++sp;
		if (sp == 4096) {
			sp = 0;
			fread(buff, 1, 4096, fin);
		}
		return buff[sp];
	}

public:
	InParser(const char* nume) {
		fin = fopen(nume, "r");
		buff = new char[4096]();
		sp = 4095;
	}

	InParser& operator >> (int &n) {
		char c;
		while (!isdigit(c = read_ch()) && c != '-');
		int sgn = 1;
		if (c == '-') {
			n = 0;
			sgn = -1;
		} else {
			n = c - '0';
		}
		while (isdigit(c = read_ch())) {
			n = 10 * n + c - '0';
		}
		n *= sgn;
		return *this;
	}

	InParser& operator >> (long long &n) {
		char c;
		n = 0;
		while (!isdigit(c = read_ch()) && c != '-');
		long long sgn = 1;
		if (c == '-') {
			n = 0;
			sgn = -1;
		} else {
			n = c - '0';
		}
		while (isdigit(c = read_ch())) {
			n = 10 * n + c - '0';
		}
		n *= sgn;
		return *this;
	}
};

InParser fin("carnati.in");
ofstream fout("carnati.out");
pair<int, int> x[2005];

int main()
{   int n, c, maxi=0, sum;
    fin>>n>>c;
    for(int i=1;i<=n;i++)
    fin>>x[i].first>>x[i].second;

    sort(x+1, x+1+n);
    for(int i=1;i<=n;i++)
    {sum=-c;
     for(int j=1;j<=n;j++)
     {if(j!=1)sum-=c*(x[j].first-x[j-1].first);

      if(sum<-c)sum=-c;
      if(x[j].second>=x[i].second)sum+=x[i].second;

      maxi=max(maxi, sum);
     }
    }

    fout<<maxi;
    return 0;
}