#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < n; i++)
#define REP(i,s,n) for(int i = s; i < n; i++)
typedef long long ll;
using namespace std;
using Graph = vector<vector<int>>;
ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
ll lcm(ll a, ll b) { return (a * b) / gcd(a,b); }
ll ceil(const ll a, const ll b) { return (a + b - 1) / b; }
int dp[1000][2];
int main(){
  int n,d;
  cin >> n >> d;
  vector<int>t(n),k(n);
  rep(i,n)cin >> t[i] >> k[i];
  dp[0][1] = -d;
  rep(i,n){
    dp[i + 1][0] = max(dp[i][0] + t[i],dp[i][1] + k[i] - d);
    dp[i + 1][1] = max(dp[i][0] + t[i] - d,dp[i][1] + k[i]);
  }
  cout << max(dp[n][0],dp[n][1]) << endl;
  return 0;
}