#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;

ll T[105], K[105];
ll dp[2][105];

int main(){
  ll N, D;
  cin >> N >> D;
  rep(i,N){
    cin >> T[i] >> K[i];
  }

  rep(i,N){
    if(i==0){
      dp[0][1] = max(dp[0][1], dp[0][0] + T[0]);
      dp[1][1] = max(dp[1][1], dp[0][0] - D + K[0]);
    }else{
      dp[0][i+1] = max(dp[0][i+1], dp[0][i] + T[i]);
      dp[1][i+1] = max(dp[1][i+1], dp[0][i] - D + K[i]);
      dp[0][i+1] = max(dp[0][i+1], dp[1][i] - D + T[i]);
      dp[1][i+1] = max(dp[1][i+1], dp[1][i] + K[i]);
    }
  }

  cout << max(dp[0][N], dp[1][N]) << endl;
  
  return 0;
}