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

typedef long long ll;
#define REP(i,n) for(int i=0,_n=(int)(n);i<_n;++i)
#define ALL(v) (v).begin(),(v).end()
template<class T1,class T2>ostream& operator<<(ostream& os,const pair<T1,T2>&a){return os<<"("<<a.first<<","<<a.second<< ")";}
template<class T>void pv(T a,T b){for(T i=a;i!=b;++i)cout<<(*i)<<" ";cout<<endl;}
template<class T>bool chmin(T&a,const T&b){return a>b?(a=b,1):0;}
template<class T>bool chmax(T&a,const T&b){return a<b?(a=b,1):0;}


int nextInt() { int x; scanf("%d", &x); return x;}

int T[128], K[128];
ll dp[128][2];
const ll INF = 1LL << 58;

int main2() {
  int N = nextInt();
  int D = nextInt();
  REP(i, N) { T[i] = nextInt(); K[i] = nextInt(); }
  REP(i, 128) REP(j, 2) dp[i][j] = -INF;

  dp[0][0] = 0;
  for (int i = 0; i < N; i++) {
    chmax(dp[i+1][0], dp[i][0] + T[i]);
    chmax(dp[i+1][0], dp[i][1] + T[i] - D);
    chmax(dp[i+1][1], dp[i][0] + K[i] - D);
    chmax(dp[i+1][1], dp[i][1] + K[i]);
  }
  ll ans = max(dp[N][0], dp[N][1]);
  cout << ans << endl;
  return 0;
}

int main() {
  for (;!cin.eof();cin>>ws) main2();
  return 0;
}