Language: C++

#include<algorithm>
#include<cmath>
#include<iomanip>
#include<iostream>
#include<map>
#include<numeric>
#include<queue>
#include<set>
#include<sstream>
#include<unordered_map>
#include<unordered_set>
#include<vector>
using ll = long long;
enum : int { M = (int)1e9 + 7 };
enum : ll { MLL = (ll)1e18L + 9 };
using namespace std;
#ifdef LOCAL
#include"rprint2.hpp"
#include"debug_deque.hpp"
#define vector DebugDeque
#else
#define FUNC(name) template <ostream& out = cout, class... T> void name(T&&...){ }
FUNC(prints) FUNC(printe) FUNC(printw) FUNC(printew) FUNC(printb) FUNC(printd) FUNC(printde);
#endif
template <class S, class T>
istream& operator >> (istream& in, pair<S, T>& p){ in >> p.first >> p.second; return in; }
template <class T>
istream& operator >> (istream& in, vector<T>& v){ for(auto& e : v){ in >> e; } return in; }

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n; cin >> n;
    vector<int> as(n); cin >> as;
    // reverse(as.begin(), as.end());
    ll num = 0, ans = 0;
    for(int i = n - 1; i >= 0; i--){
        if(as[i] > i || num > as[i]){
            cout << -1 << '\n';
            return 0;
        }
        if(as[i] != num){
            ans += as[i];
        }
        prints(i, ans);
        num = as[i];
        num = max(num - 1, 0);
    }
    cout << ans << '\n';
}


Compiler output:
a.cc: In function 'int main()':
a.cc:47:18: error: no matching function for call to 'max(ll, int)'
   47 |         num = max(num - 1, 0);
      |               ~~~^~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 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:47:18: note:   deduced conflicting types for parameter 'const _Tp' ('long long int' and 'int')
   47 |         num = max(num - 1, 0);
      |               ~~~^~~~~~~~~~~~
/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:
/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:47:18: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   47 |         num = max(num - 1, 0);
      |               ~~~^~~~~~~~~~~~