Language: C++

#include <iostream>
#include <utility>
#include <tuple>
#include <vector>
#include <string>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <algorithm>
#include <functional>
#include <climits>
#include <numeric>
#include <queue>
#include <cmath>
#include <iomanip>
#include <array>
#include <string>
#include <stack>
#include <cassert>
#include <memory>
#include <random>

int main() {
	int n, m, k; std::cin >> n >> m >> k;
	std::vector<long long int> a_books(n), b_books(m);
	for (auto& a : a_books) std::cin >> a;
	for (auto& b : b_books) std::cin >> b;
	a_books.insert(a_books.begin(), 0);
	b_books.insert(b_books.begin(), 0);
	for (auto i = 1; i < a_books.size(); ++i) {
		a_books[i] += a_books[i - 1];
	}
	for (auto i = 1; i < b_books.size(); ++i) {
		b_books[i] += b_books[i - 1];
	}
	int max_count{ 0 };
	for (auto i = 0; i < a_books.size(); ++i) {
		if (a_books[i] > k) break;
		const auto j = std::distance(b_books.begin(), std::prev(std::upper_bound(b_books.begin(), b_books.end(), k - a_books[i])));
		max_count = std::max(max_count, i + j);
	}
	std::cout << max_count << '\n';
}

Compiler output:
a.cc: In function 'int main()':
a.cc:41:37: error: no matching function for call to 'max(int&, long int)'
   41 |                 max_count = std::max(max_count, i + j);
      |                             ~~~~~~~~^~~~~~~~~~~~~~~~~~
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:41:37: note:   deduced conflicting types for parameter 'const _Tp' ('int' and 'long int')
   41 |                 max_count = std::max(max_count, i + j);
      |                             ~~~~~~~~^~~~~~~~~~~~~~~~~~
/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:10:
/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:41:37: note:   mismatched types 'std::initializer_list<_Tp>' and 'int'
   41 |                 max_count = std::max(max_count, i + j);
      |                             ~~~~~~~~^~~~~~~~~~~~~~~~~~