#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long int li;
typedef long double lb;
#define rep(i,j,n) for (ll i = j; i < (n); i++)
#define repr(i,j,n) for(ll i = j; i >= (n); i--)
#define all(x) (x).begin(),(x).end()
#define CLR(mat,f) memset(mat, f, sizeof(mat))
#define IN(a, b, x) (a<=x&&x<b)
#define out(ans) cout << ans << endl
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
typedef pair<ll,ll>P;
const ll mod=1e9+7;
const int INF = 1000000;
const double PI=3.14159265359;

int main(){

  ll n,d;cin>>n>>d;
  ll t[n],k[n];rep(i,0,n)cin>>t[i]>>k[i];

  // rep(i,1,n)dp[0][i]=t[i]+dp[0][i-1];
  // CLR(dp[0],0);
  // CLR(dp[1],0);
  ll dp[2][n];
  dp[0][0]=t[0];
  dp[1][0]=k[0]-d;
  rep(i,1,n){
    dp[0][i]=max(dp[0][i-1]+t[i],dp[1][i-1]+t[i]-d);
    dp[1][i]=max(dp[1][i-1]+k[i],dp[0][i-1]+k[i]-d);
  }

  out(max(dp[0][n-1],dp[1][n-1]));


  return 0;
}