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

#define int long long

#define all(v) (v).begin(),(v).end()
#define pb push_back
#define fi first
#define se second
#define rep(i,s,n) for(int i = s;i<n;i++)
#define repe(i,s,n) for(int i = s;i<=n;i++)
#define rrep(i,s,n) for(int i = (n)-1;i>=(s);i--)
#define chmin(a,b) a=min((a),(b))
#define chmax(a,b) a=max((a),(b))
#define low(a,b) lower_bound(all(a),(b))
#define upp(a,b) upper_bound(all(a),(b))
#define fill(i,n) memset(i,n,sizeof(i)
#define endl '\n'
#define IOS() ios_base::sync_with_stdio(0);cin.tie(0)
typedef long long ll;
typedef pair<int,int>pint;
typedef vector<int>vint;
typedef vector<pint>vpint;
typedef pair<pint,int> P1;
typedef pair<int,pint> P2;
typedef pair<pint,pint> PP;
static const ll maxLL = (ll)1 << 62;
const ll MOD=1000000007;
const ll INF=1e18;

int N,D;
int t[100010],k[100010];
int dp[100010][2];

signed main() {
    IOS();
    /*シフト演算は(1ll<<x)*/
    
    cin>>N>>D;
    rep(i,0,N)cin>>t[i]>>k[i];
    
    rep(i,0,N){
        dp[i+1][1]=max(dp[i][1],dp[i][2]-D)+t[i];
        if(i==0)dp[i+1][2]=dp[i][1]-D+k[i];
        else dp[i+1][2]=max(dp[i][2],dp[i][1]-D)+k[i];
        //cout<<dp[i+1][1]<<" "<<dp[i+1][2]<<endl;
    }
    cout<<max(dp[N][1],dp[N][2])<<endl;
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    return 0;
}