#include<bits/stdc++.h>
#define REP(i,n) for(int i=0;i<n;i++)
#define LOOP(i,x,n) for(int i=x;i<n;i++)
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define MP make_pair
#define FR first
#define SC second
#define int long long
using namespace std;
const int MOD=1000000007;
const int INF=9000000009;

int dp[100000000][2];

signed main(){
   int n,d;
   cin>>n>>d;
   vector<int> t(n),k(n);
   REP(i,n)cin>>t[i]>>k[i];
   dp[0][0]=t[0];
   dp[0][1]=(k[0]-d>=0?k[0]-d:0);
   LOOP(i,1,n){
     dp[i][0]+=t[i]+max(dp[i-1][0],dp[i-1][1]-d);
     dp[i][1]+=k[i]+max(dp[i-1][0]-d,dp[i-1][1]);
   }
   cout<<max(dp[n-1][0],dp[n-1][1])<<endl;
  return 0;
}