#include <iostream>
#include <fstream>
#include <algorithm>
#define MAX 2002
using namespace std;
pair<int, int> v[MAX],t[MAX];
int n,dp[MAX],c,ans;

ifstream fin("carnati.in");
ofstream fout("carnati.out");

int main()
{
    fin >> n >> c;
    for(int i = 1; i <= n; i++){
        fin >> v[i].first >> v[i].second;
    }
    sort(v+1, v+n+1);
    for(int i = 1; i <= n; i++){
        /// fixam p ca fiind v[i].second
        int cnt = 0;
        for(int j = 1; j <= n; j++){
            if(v[j].second >= v[i].second){
                t[++cnt] = {v[j].first, v[j].second};
            }
        }
        dp[1] = v[i].second-c;
        ans = max(ans, dp[1]);
        for(int j = 2; j <= cnt; j++){
            dp[j] = max( v[i].second-c, dp[j-1]+v[i].second-c*(t[j].first-t[j-1].first) );
            ans = max(ans, dp[j]);
        }
    }
    fout << ans;
    return 0;
}