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#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx")
#pragma GCC optimize("unroll-loops")    
#include <bits/stdc++.h>
#define pb push_back
#define x first
#define y second
#define mp make_pair
#define len(a) int(a.size())
#define files(FILENAME) read(FILENAME); write(FILENAME)
#define read(FILENAME) freopen((string(FILENAME) + ".in").c_str(), "r", stdin)
#define write(FILENAME) freopen((string(FILENAME) + ".out").c_str(), "w", stdout)
using namespace std;
        
template<typename T1, typename T2>inline void chkmin(T1 &x, T2 y) { if (x > y) x = y; }
template<typename T1, typename T2>inline void chkmax(T1 &x, T2 y) { if (x < y) x = y; } 
template<typename T, typename U> inline ostream &operator<< (ostream &_out, const pair<T, U> &_p) { _out << _p.first << ' ' << _p.second; return _out; }
template<typename T, typename U> inline istream &operator>> (istream &_in, pair<T, U> &_p) { _in >> _p.first >> _p.second; return _in; }
template<typename T> inline ostream &operator<< (ostream &_out, const vector<T> &_v) { if (_v.empty()) { return _out; } _out << _v.front(); for (auto _it = ++_v.begin(); _it != _v.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T> inline istream &operator>> (istream &_in, vector<T> &_v) { for (auto &_i : _v) { _in >> _i; } return _in; }
template<typename T> inline ostream &operator<< (ostream &_out, const set<T> &_s) { if (_s.empty()) { return _out; } _out << *_s.begin(); for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T> inline ostream &operator<< (ostream &_out, const multiset<T> &_s) { if (_s.empty()) { return _out; } _out << *_s.begin(); for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T> inline ostream &operator<< (ostream &_out, const unordered_set<T> &_s) { if (_s.empty()) { return _out; } _out << *_s.begin(); for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T> inline ostream &operator<< (ostream &_out, const unordered_multiset<T> &_s) { if (_s.empty()) { return _out; } _out << *_s.begin(); for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) { _out << ' ' << *_it; } return _out; }
template<typename T, typename U> inline ostream &operator<< (ostream &_out, const map<T, U> &_m) { if (_m.empty()) { return _out; } _out << '(' << _m.begin()->first << ": " << _m.begin()->second << ')'; for (auto _it = ++_m.begin(); _it != _m.end(); ++_it) { _out << ", (" << _it->first << ": " << _it->second << ')'; } return _out; }
template<typename T, typename U> inline ostream &operator<< (ostream &_out, const unordered_map<T, U> &_m) { if (_m.empty()) { return _out; } _out << '(' << _m.begin()->first << ": " << _m.begin()->second << ')'; for (auto _it = ++_m.begin(); _it != _m.end(); ++_it) { _out << ", (" << _it->first << ": " << _it->second << ')'; } return _out; }
        
typedef long long base; 
typedef pair<int, int> point;      
typedef complex<double> comp;

const int N = 1e6 + 5;

int n;
int ans = 0;
int a[N], b[N];
set<pair<int, int>> L[2], R[2];
vector<pair<int, int>> start;
vector<pair<int, int>> finish;

int get(int t) {
    if (L[t].empty()) return 0;
    return max(0, R[t].begin()->x - L[t].rbegin()->x);
}

vector<int> id;

int main() {
    ios::sync_with_stdio(0);
    srand(time(0));
#ifdef ONPC
    freopen("input", "r", stdin);
#endif
    cin >> n;
    for (int i = 0; i < n; ++i) {
        cin >> a[i] >> b[i];
        --a[i];
        start.emplace_back(a[i], i);
        finish.emplace_back(b[i], i);
        L[1].insert({a[i], i});
        R[1].insert({b[i], i});
        id.pb(i);
        chkmax(ans, b[i] - a[i]);
    }
    sort(id.begin(), id.end(), [](int i, int j) {
        if (a[i] != a[j]) {
            return a[i] < a[j];
        }
        return b[i] < b[j];
    });
    for (int i : id) {
        L[0].insert({a[i], i});
        L[1].erase({a[i], i});
        R[0].insert({b[i], i});
        R[1].erase({b[i], i});
        chkmax(ans, get(0) + get(1));
    }
    for (int i : id) {
        L[1].insert({a[i], i});
        L[0].erase({a[i], i});
        R[1].insert({b[i], i});
        R[0].erase({b[i], i});
        chkmax(ans, get(0) + get(1));
        L[0].insert({a[i], i});
        L[1].erase({a[i], i});
        R[0].insert({b[i], i});
        R[1].erase({b[i], i});

    }
    cout << ans << '\n';
}  



Measured memory_bytes: 16104