#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef pair<int,int> P;

int main(){
  int n;
  ll D;
  cin >> n >> D;
  vector<ll> t(n), k(n);
  rep(i,n) cin >> t[i] >> k[i];

  vector<vector<ll>> dp(n+1,vector<ll>(2,0));

  rep(i,n) {
    dp[i+1][0] = max(dp[i+1][0], max(dp[i][0] + t[i], dp[i][1] + t[i] - D));
    if (i > 0) dp[i+1][1] = max(dp[i+1][1], max(dp[i][1] + k[i], dp[i][0] + k[i] - D));
    else dp[i+1][1] = dp[i][1] + k[i] - D;
  }
  cout << max(dp[n][0], dp[n][1]) << endl;
  return 0;
}