Language: C++

#include <iostream>
#include <algorithm>
#include <set>
#include <vector>
#include <map>

using namespace std;
#define int long long

int n;
pair<int, int> segms[500000];

#define l first
#define r second

bool comp(pair<int, int> f, pair<int, int> g)
{
    return f.second < g.second;
}

int len1[500000];

signed main()
{
    cin >> n;
    for (int i = 0; i < n; i++) cin >> segms[i].l >> segms[i].r;
    sort(segms, segms + n);
    int l2 = segms[n - 1].first;
    int ans = 0;
    for (int i = 0; i < n; i++) ans = max(ans, segms[i].r - segms[i].l + 1);
    sort(segms, segms + n, comp);
    int l1 = segms[0].first, r1 = segms[0].second;
    for (int j = 0; j < n; j++)
    {
        l1 = max(l1, segms[j].first);
        r1 = min(r1, segms[j].second);
        len1[j] = max(0, r1 - l1 + 1);
    }
    for (int j = n - 1; j >= 0; j--)
    {
        int r2 = segms[j].second;
        int len2 = r2 - l2 + 1;
        if (len2 < 0) break;
        //cout << len1[j - 1] << " " << len2 << endl;
        ans = max(ans, (j > 0 ? len1[j - 1] : 0) + len2);
    }
    ans = max(ans, len1[n - 2] + segms[n - 1].r - segms[n - 1].l + 1);
    cout << ans;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:37:22: error: no matching function for call to 'max(int, long long int)'
   37 |         len1[j] = max(0, r1 - l1 + 1);
      |                   ~~~^~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/string:51,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:37:22: note:   deduced conflicting types for parameter 'const _Tp' ('int' and 'long long int')
   37 |         len1[j] = max(0, r1 - l1 + 1);
      |                   ~~~^~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:37:22: note:   mismatched types 'std::initializer_list<_Tp>' and 'int'
   37 |         len1[j] = max(0, r1 - l1 + 1);
      |                   ~~~^~~~~~~~~~~~~~~~