Language: C++

#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#define FOR(i, n, m) for(ll i = n; i < (int)m; i++)
#define REP(i, n) FOR(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define pb push_back
using namespace std;
using ll = std::int_fast64_t;
using P = pair<ll, ll>;
constexpr ll inf = 1000000000;
constexpr ll mod = 1000000007;
constexpr long double eps = 1e-15;

template<typename T1, typename T2>
ostream& operator<<(ostream& os, pair<T1, T2> p) {
    os << to_string(p.first) << " " << to_string(p.second);
    return os;
}
template<typename T>
ostream& operator<<(ostream& os, vector<T>& v) {
    REP(i, v.size()) {
        if(i) os << " ";
        os << to_string(v[i]);
    }
    return os;
}

struct modint {
    ll n;
public:
    modint(const ll n = 0) : n((n % mod + mod) % mod) {}
    static modint pow(modint a, int m) {
        modint r = 1;
        while(m > 0) {
            if(m & 1) { r *= a; }
            a = (a * a); m /= 2;
        }
        return r;
    }
    modint &operator++() { *this += 1; return *this; }
    modint &operator--() { *this -= 1; return *this; }
    modint operator++(int) { modint ret = *this; *this += 1; return ret; }
    modint operator--(int) { modint ret = *this; *this -= 1; return ret; }
    modint operator~() const { return (this -> pow(n, mod - 2)); } // inverse
    friend bool operator==(const modint& lhs, const modint& rhs) {
        return lhs.n == rhs.n;
    }
    friend bool operator<(const modint& lhs, const modint& rhs) {
        return lhs.n < rhs.n;
    }
    friend bool operator>(const modint& lhs, const modint& rhs) {
        return lhs.n > rhs.n;
    }
    friend modint &operator+=(modint& lhs, const modint& rhs) {
        lhs.n += rhs.n;
        if (lhs.n >= mod) lhs.n -= mod;
        return lhs;
    }
    friend modint &operator-=(modint& lhs, const modint& rhs) {
        lhs.n -= rhs.n;
        if (lhs.n < 0) lhs.n += mod;
        return lhs;
    }
    friend modint &operator*=(modint& lhs, const modint& rhs) {
        lhs.n = (lhs.n * rhs.n) % mod;
        return lhs;
    }
    friend modint &operator/=(modint& lhs, const modint& rhs) {
        lhs.n = (lhs.n * (~rhs).n) % mod;
        return lhs;
    }
    friend modint operator+(const modint& lhs, const modint& rhs) {
        return modint(lhs.n + rhs.n);
    }
    friend modint operator-(const modint& lhs, const modint& rhs) {
        return modint(lhs.n - rhs.n);
    }
    friend modint operator*(const modint& lhs, const modint& rhs) {
        return modint(lhs.n * rhs.n);
    }
    friend modint operator/(const modint& lhs, const modint& rhs) {
        return modint(lhs.n * (~rhs).n);
    }
};
istream& operator>>(istream& is, modint m) { is >> m.n; return is; }
ostream& operator<<(ostream& os, modint m) { os << m.n; return os; }

#define MAX_N 3030303
long long extgcd(long long a, long long b, long long& x, long long& y) {
    long long d = a;
    if (b != 0) {
        d = extgcd(b, a % b, y, x);
        y -= (a / b) * x;
    } else {
        x = 1; y = 0;
    }
    return d;
}
long long mod_inverse(long long a, long long m) {
    long long x, y;
    if(extgcd(a, m, x, y) == 1) return (m + x % m) % m;
    else return -1;
}
vector<long long> fact(MAX_N+1, inf);
long long mod_fact(long long n, long long& e) {
    if(fact[0] == inf) {
        fact[0]=1;
        if(MAX_N != 0) fact[1]=1;
        for(ll i = 2; i <= MAX_N; ++i) {
            fact[i] = (fact[i-1] * i) % mod;
        }
    }
    e = 0;
    if(n == 0) return 1;
    long long res = mod_fact(n / mod, e);
    e += n / mod;
    if((n / mod) % 2 != 0) return (res * (mod - fact[n % mod])) % mod;
    return (res * fact[n % mod]) % mod;
}
// return nCk
long long mod_comb(long long n, long long k) {
    if(n < 0 || k < 0 || n < k) return 0;
    long long e1, e2, e3;
    long long a1 = mod_fact(n, e1), a2 = mod_fact(k, e2), a3 = mod_fact(n - k, e3);
    if(e1 > e2 + e3) return 0;
    return (a1 * mod_inverse((a2 * a3) % mod, mod)) % mod;
}

using mi = modint;

mi mod_pow(ll a, ll n) {
    mi ret = 1;
    mi tmp = a;
    while(n > 0) {
        if(n % 2) ret *= tmp;
        tmp = tmp * tmp;
        n /= 2;
    }
    return ret;
}

ll calc(vector<P> v) {
    int n = (int)v.size();
    vector<P> mm1(n), mm2(n);
    mm1[0] = v[0];
    mm2[n - 1] = v[n - 1];
    FOR(i, 1, n) {
        mm1[i].first = max(mm1[i - 1].first, v[i].first);
        mm1[i].second = min(mm1[i - 1].second, v[i].second);
    }
    for(int i = n - 2; i >= 0; i--) {
        mm2[i].first = max(mm2[i + 1].first, v[i].first);
        mm2[i].second = min(mm2[i + 1].second, v[i].second);
    }
    ll ret = 0;
    if(n == 1) return v[0].second - v[0].first + 1;
    REP(i, n) {
        ll tmp = v[i].second - v[i].first + 1;
        P mm = {0, inf};
        if(i) {
            mm.first = max(mm.first, mm1[i - 1].first);
            mm.second = min(mm.second, mm1[i - 1].second);
        }
        if(i < n - 1) {
            mm.first = max(mm.first, mm2[i + 1].first);
            mm.second = min(mm.second, mm2[i + 1].second);
        }
        tmp += max(0LL, mm.second - mm.first + 1);
        ret = max(ret, tmp);
    }
    return ret;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    vector<P> v(n);
    REP(i, n) {
        cin >> v[i].first >> v[i].second;
    }
    ll ans = calc(v);
    REP(i, n) v[i].second = -v[i].second;
    sort(ALL(v));
    REP(i, n) v[i].second = -v[i].second;
    vector<P> nv;
    ll mn = v[n - 1].second;
    nv.pb(v[n - 1]);
    for(int i = n - 2; i >= 0; i--) {
        if(v[i].second < mn) {
            mn = v[i].second;
            nv.pb(v[i]);
        }
    }
    sort(ALL(nv));
    int nn = (int)nv.size();
    ans = max(ans, calc(nv));
    REP(i, nn - 1) {
        ll tmp1 = nv[0].second - nv[i].first + 1;
        if(tmp1 <= 0) continue;
        ll tmp2 = nv[i + 1].second - nv[nn - 1].first + 1;
        if(tmp2 <= 0) continue;
        ans = max(ans, tmp1 + tmp2);
    }
    cout << ans << endl;
    return 0;
}


Compiler output:
a.cc:12:17: error: 'int_fast64_t' in namespace 'std' does not name a type
   12 | using ll = std::int_fast64_t;
      |                 ^~~~~~~~~~~~
a.cc:13:16: error: 'll' was not declared in this scope
   13 | using P = pair<ll, ll>;
      |                ^~
a.cc:13:20: error: 'll' was not declared in this scope
   13 | using P = pair<ll, ll>;
      |                    ^~
a.cc:13:22: error: template argument 1 is invalid
   13 | using P = pair<ll, ll>;
      |                      ^
a.cc:13:22: error: template argument 2 is invalid
a.cc:14:11: error: 'll' does not name a type
   14 | constexpr ll inf = 1000000000;
      |           ^~
a.cc:15:11: error: 'll' does not name a type
   15 | constexpr ll mod = 1000000007;
      |           ^~
a.cc: In function 'std::ostream& operator<<(std::ostream&, std::vector<_Tp>&)':
a.cc:7:26: error: 'll' was not declared in this scope
    7 | #define FOR(i, n, m) for(ll i = n; i < (int)m; i++)
      |                          ^~
a.cc:8:19: note: in expansion of macro 'FOR'
    8 | #define REP(i, n) FOR(i, 0, n)
      |                   ^~~
a.cc:25:5: note: in expansion of macro 'REP'
   25 |     REP(i, v.size()) {
      |     ^~~
a.cc:25:9: error: 'i' was not declared in this scope
   25 |     REP(i, v.size()) {
      |         ^
a.cc:7:36: note: in definition of macro 'FOR'
    7 | #define FOR(i, n, m) for(ll i = n; i < (int)m; i++)
      |                                    ^
a.cc:25:5: note: in expansion of macro 'REP'
   25 |     REP(i, v.size()) {
      |     ^~~
a.cc: At global scope:
a.cc:33:5: error: 'll' does not name a type
   33 |     ll n;
      |     ^~
a.cc:35:18: error: 'll' does not name a type
   35 |     modint(const ll n = 0) : n((n % mod + mod) % mod) {}
      |                  ^~
a.cc: In constructor 'modint::modint(int)':
a.cc:35:30: error: class 'modint' does not have any field named 'n'
   35 |     modint(const ll n = 0) : n((n % mod + mod) % mod) {}
      |                              ^
a.cc:35:37: error: 'mod' was not declared in this scope
   35 |     modint(const ll n = 0) : n((n % mod + mod) % mod) {}
      |                                     ^~~
a.cc: In member function 'modint modint::operator~() const':
a.cc:48:52: error: 'n' was not declared in this scope
   48 |     modint operator~() const { return (this -> pow(n, mod - 2)); } // inverse
      |                                                    ^
a.cc:48:55: error: 'mod' was not declared in this scope
   48 |     modint operator~() const { return (this -> pow(n, mod - 2)); } // inverse
      |                                                       ^~~
a.cc: In function 'bool operator==(const modint&, const modint&)':
a.cc:50:20: error: 'const struct modint' has no member named 'n'
   50 |         return lhs.n == rhs.n;
      |                    ^
a.cc:50:29: error: 'const struct modint' has no member named 'n'
   50 |         return lhs.n == rhs.n;
      |                             ^
a.cc: In function 'bool operator<(const modint&, const modint&)':
a.cc:53:20: error: 'const struct modint' has no member named 'n'
   53 |         return lhs.n < rhs.n;
      |                    ^
a.cc:53:28: error: 'const struct modint' has no member named 'n'
   53 |         return lhs.n < rhs.n;
      |                            ^
a.cc: In function 'bool operator>(const modint&, const modint&)':
a.cc:56:20: error: 'const struct modint' has no member named 'n'
   56 |         return lhs.n > rhs.n;
      |                    ^
a.cc:56:28: error: 'const struct modint' has no member named 'n'
   56 |         return lhs.n > rhs.n;
      |                            ^
a.cc: In function 'modint& operator+=(modint&, const modint&)':
a.cc:59:13: error: 'struct modint' has no member named 'n'
   59 |         lhs.n += rhs.n;
      |             ^
a.cc:59:22: error: 'const struct modint' has no member named 'n'
   59 |         lhs.n += rhs.n;
      |                      ^
a.cc:60:17: error: 'struct modint' has no member named 'n'
   60 |         if (lhs.n >= mod) lhs.n -= mod;
      |                 ^
a.cc:60:22: error: 'mod' was not declared in this scope
   60 |         if (lhs.n >= mod) lhs.n -= mod;
      |                      ^~~
a.cc:60:31: error: 'struct modint' has no member named 'n'
   60 |         if (lhs.n >= mod) lhs.n -= mod;
      |                               ^
a.cc: In function 'modint& operator-=(modint&, const modint&)':
a.cc:64:13: error: 'struct modint' has no member named 'n'
   64 |         lhs.n -= rhs.n;
      |             ^
a.cc:64:22: error: 'const struct modint' has no member named 'n'
   64 |         lhs.n -= rhs.n;
      |                      ^
a.cc:65:17: error: 'struct modint' has no member named 'n'
   65 |         if (lhs.n < 0) lhs.n += mod;
      |                 ^
a.cc:65:28: error: 'struct modint' has no member named 'n'
   65 |         if (lhs.n < 0) lhs.n += mod;
      |                            ^
a.cc:65:33: error: 'mod' was not declared in this scope
   65 |         if (lhs.n < 0) lhs.n += mod;
      |                                 ^~~
a.cc: In function 'modint& operator*=(modint&, const modint&)':
a.cc:69:13: error: 'struct modint' has no member named 'n'
   69 |         lhs.n = (lhs.n * rhs.n) % mod;
      |             ^
a.cc:69:22: error: 'struct modint' has no member named 'n'
   69 |         lhs.n = (lhs.n * rhs.n) % mod;
      |                      ^
a.cc:69:30: error: 'const struct modint' has no member named 'n'
   69 |         lhs.n = (lhs.n * rhs.n) % mod;
      |                              ^
a.cc:69:35: error: 'mod' was not declared in this scope
   69 |         lhs.n = (lhs.n * rhs.n) % mod;
      |                                   ^~~
a.cc: In function 'modint& operator/=(modint&, const modint&)':
a.cc:73:13: error: 'struct modint' has no member named 'n'
   73 |         lhs.n = (lhs.n * (~rhs).n) % mod;
      |             ^
a.cc:73:22: error: 'struct modint' has no member named 'n'
   73 |         lhs.n = (lhs.n * (~rhs).n) % mod;
      |                      ^
a.cc:73:33: error: 'struct modint' has no member named 'n'
   73 |         lhs.n = (lhs.n * (~rhs).n) % mod;
      |                                 ^
a.cc:73:38: error: 'mod' was not declared in this scope
   73 |         lhs.n = (lhs.n * (~rhs).n) % mod;
      |                                      ^~~
a.cc: In function 'modint operator+(const modint&, const modint&)':
a.cc:77:27: error: 'const struct modint' has no member named 'n'
   77 |         return modint(lhs.n + rhs.n);
      |                           ^
a.cc:77:35: error: 'const struct modint' has no member named 'n'
   77 |         return modint(lhs.n + rhs.n);
      |                                   ^
a.cc: In function 'modint operator-(const modint&, const modint&)':
a.cc:80:27: error: 'const struct modint' has no member named 'n'
   80 |         return modint(lhs.n - rhs.n);
      |                           ^
a.cc:80:35: error: 'const struct modint' has no member named 'n'
   80 |         return modint(lhs.n - rhs.n);
      |                                   ^
a.cc: In function 'modint operator*(const modint&, const modint&)':
a.cc:83:27: error: 'const struct modint' has no member named 'n'
   83 |         return modint(lhs.n * rhs.n);
      |                           ^
a.cc:83:35: error: 'const struct modint' has no member named 'n'
   83 |         return modint(lhs.n * rhs.n);
      |                                   ^
a.cc: In function 'modint operator/(const modint&, const modint&)':
a.cc:86:27: error: 'const struct modint' has no member named 'n'
   86 |         return modint(lhs.n * (~rhs).n);
      |                           ^
a.cc:86:38: error: 'struct modint' has no member named 'n'
   86 |         return modint(lhs.n * (~rhs).n);
      |                                      ^
a.cc: In function 'std::istream& operator>>(std::istream&, modint)':
a.cc:89:54: error: 'struct modint' has no member named 'n'
   89 | istream& operator>>(istream& is, modint m) { is >> m.n; return is; }
      |                                                      ^
a.cc: In function 'std::ostream& operator<<(std::ostream&, modint)':
a.cc:90:54: error: 'struct modint' has no member named 'n'
   90 | ostream& operator<<(ostream& os, modint m) { os << m.n; return os; }
      |                                                      ^
a.cc: At global scope:
a.cc:108:33: error: 'inf' was not declared in this scope; did you mean 'int'?
  108 | vector<long long> fact(MAX_N+1, inf);
      |                                 ^~~
      |                                 int
a.cc: In function 'long long int mod_fact(long long int, long long int&)':
a.cc:110:19: error: 'inf' was not declared in this scope; did you mean 'int'?
  110 |     if(fact[0] == inf) {
      |                   ^~~
      |                   int
a.cc:113:13: error: 'll' was not declared in this scope
  113 |         for(ll i = 2; i <= MAX_N; ++i) {
      |             ^~
a.cc:113:23: error: 'i' was not declared in this scope
  113 |         for(ll i = 2; i <= MAX_N; ++i) {
      |                       ^
a.cc:114:41: error: 'mod' was not declared in this scope
  114 |             fact[i] = (fact[i-1] * i) % mod;
      |                                         ^~~
a.cc:119:34: error: 'mod' was not declared in this scope
  119 |     long long res = mod_fact(n / mod, e);
      |                                  ^~~
a.cc: In function 'long long int mod_comb(long long int, long long int)':
a.cc:130:42: error: 'mod' was not declared in this scope
  130 |     return (a1 * mod_inverse((a2 * a3) % mod, mod)) % mod;
      |                                          ^~~
a.cc: At global scope:
a.cc:135:12: error: 'll' was not declared in this scope
  135 | mi mod_pow(ll a, ll n) {
      |            ^~
a.cc:135:18: error: 'll' was not declared in this scope
  135 | mi mod_pow(ll a, ll n) {
      |                  ^~
a.cc:135:24: error: expected ',' or ';' before '{' token
  135 | mi mod_pow(ll a, ll n) {
      |                        ^
a.cc:146:1: error: 'll' does not name a type
  146 | ll calc(vector<P> v) {
      | ^~
a.cc: In function 'int main()':
a.cc:183:12: error: 'P' was not declared in this scope
  183 |