#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,j,n) for(ll i=j; n>i; ++i)
const ll inf = -1e12;

int main(){
    ll n,d; cin >> n >> d;
    vector<ll> t(n),k(n);
    rep(i,0,n) cin >> t[i] >> k[i];
    //dp[n][city] := n日目にcityにいるときの最大値
    //0 -> tokyo, 1-> kyoto;
    ll dp[n+1][2];
    for(ll i=0; n>=i; ++i){
        for(ll j=0; 2>j; ++j){
            dp[i][j] = inf;
        }
    }
    dp[0][0] = 0;
    for(ll i=1; n>=i; ++i){
        for(ll j=0; 2>j; ++j){
            dp[i][j] = max(dp[i-1][j],dp[i-1][1-j]-d);
            if(j == 0) dp[i][j] += t[i-1];
            else dp[i][j] += k[i-1];
        }
    }
    cout << max(dp[n][0],dp[n][1]) << endl;
    /*for(int i=0; 2>i; ++i){
        for(int j=0; n>=j; ++j){
            cout << dp[j][i] << " ";
        }
        cout << endl;
    }
    cout << endl;*/
}