#include <stdio.h>
#include <algorithm>
using namespace std;

struct om{
    int first,second;
}p[2005];

bool comp(om o1, om o2){
    if(o1.first == o2.first){
        return o1.second < o2.second;
    }
    return o1.first < o2.first;
}

int main(){
    freopen("carnati.in", "r", stdin);
    freopen("carnati.out", "w", stdout);
    int n,i,j,sum,bst,C,id;
    bst = 0;
    scanf("%d %d",&n,&C);
    for(i = 1;i <= n;i++){
        scanf("%d %d",&p[i].first, &p[i].second);
    }
    sort(p+1, p+n+1, comp);
    for(i = 1;i <= n;i++){
        int P = p[i].second;
        sum = 0;
        id = 1;
        for(j = 1;j <= n;j++){
            if(p[j].second >= P){
                sum += P;
            }
            bst = max(bst, sum-C*(p[j].first - p[id].first + 1));
            if(sum-C*(p[j].first-p[id].first+1) < 0){
                sum = 0;
                id = j+1;
                if(p[j].second >= P && P-C > 0){
                    sum = P;
                    id = j;
                    bst = max(bst, sum-C);
                }
            }else if(sum-C*(p[j].first-p[id].first+1) < P-C && p[j].second >= P){
                sum = P;
                id = j;
                bst = max(bst, sum-C);
            }else{
                bst = max(bst, sum-C*(p[j].first-p[id].first+1));
            }
        }
    }
    printf("%d",bst);
    return 0;
}