#include <bits/stdc++.h>
using namespace std;
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
#endif
using ll = long long;
using ld = long double;
const ll INF = 1ll<<60;
const ld EPS = 1.0/1e9;
#define endl "\n"
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)
#define fore(i,a) for(auto &i:a)
#define all(x) (x).begin(),(x).end()
#define del(x) sort(all(x)); x.erase(unique(all(x)),x.end());

int main(){
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  ll n,d; cin >> n >> d;
  vector<ll>p(n);
  vector<ll>q(n);
  rep(i,0,n) cin >> p[i] >> q[i];
  vector<pair<ll,ll>>pq;
  rep(i,0,n){
    pq.emplace_back(p[i]-q[i],p[i]);
  }
  vector<vector<pair<ll,ll>>>dp(2,vector<pair<ll,ll>>(n,{-INF,-INF}));
  rep(i,0,n){
    dp[0][i].first=-pq[i].second;
    dp[0][i].second=-pq[i].first;
  }
  rep(i,0,n){
    ll x=-INF; ll ind=-1;
    rep(j,0,n){
      if(x<dp[0][i].first-pq[j].second){
        x=max(x,dp[0][i].first-pq[j].second);
        ind=j;
      }
    }
    dp[1][i].first=x;
    dp[1][i].second=dp[0][i].second-pq[ind].first;
  }
  ll tmp=-INF; ll tmp2=-INF;
  rep(i,0,n){
    tmp=max(tmp,min(dp[1][i].first,dp[1][i].second));
  }
  cout << tmp << endl;
}