#include<bits/stdc++.h>
using namespace std;

int main(){
    int n,d;
    cin >> n >> d;

    int tokyo = 0,kyoto = INT_MIN;
    
    for(int i = 0;i<n;i++){
        int a,b;
        cin >> a >> b;
        int nt,nk;
        nt = max(tokyo+a, kyoto+a-d);
        nk = max(tokyo+b-d,kyoto+b);
        tokyo = nt;kyoto = nk;
    }

    cout << max(tokyo,kyoto) << endl;
    
    
}