Language: C++

#include <bits/stdc++.h>
using namespace std;

int main() {
 int N;
 string L(86, '1');
  cin >> N;
  L.at(0) = "2";
  L.at(1) = "1";
  for(int i = 2; i < N; i++){
    int b = stoi(L.at(i - 1));
    int c = stoi(L.at(i - 2));
    L.at(i) = a + b;
   }
  cout << L.at(N) << endl;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:8:13: error: invalid conversion from 'const char*' to '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} [-fpermissive]
    8 |   L.at(0) = "2";
      |             ^~~
      |             |
      |             const char*
a.cc:9:13: error: invalid conversion from 'const char*' to '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} [-fpermissive]
    9 |   L.at(1) = "1";
      |             ^~~
      |             |
      |             const char*
a.cc:11:17: error: no matching function for call to 'stoi(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&)'
   11 |     int b = stoi(L.at(i - 1));
      |             ~~~~^~~~~~~~~~~~~
In file included from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.h:4164:3: note: candidate: 'int std::__cxx11::stoi(const std::string&, std::size_t*, int)'
 4164 |   stoi(const string& __str, size_t* __idx = 0, int __base = 10)
      |   ^~~~
/usr/include/c++/14/bits/basic_string.h:4164:22: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const std::string&' {aka 'const std::__cxx11::basic_string<char>&'}
 4164 |   stoi(const string& __str, size_t* __idx = 0, int __base = 10)
      |        ~~~~~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:4432:3: note: candidate: 'int std::__cxx11::stoi(const std::wstring&, std::size_t*, int)'
 4432 |   stoi(const wstring& __str, size_t* __idx = 0, int __base = 10)
      |   ^~~~
/usr/include/c++/14/bits/basic_string.h:4432:23: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const std::wstring&' {aka 'const std::__cxx11::basic_string<wchar_t>&'}
 4432 |   stoi(const wstring& __str, size_t* __idx = 0, int __base = 10)
      |        ~~~~~~~~~~~~~~~^~~~~
a.cc:12:17: error: no matching function for call to 'stoi(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&)'
   12 |     int c = stoi(L.at(i - 2));
      |             ~~~~^~~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:4164:3: note: candidate: 'int std::__cxx11::stoi(const std::string&, std::size_t*, int)'
 4164 |   stoi(const string& __str, size_t* __idx = 0, int __base = 10)
      |   ^~~~
/usr/include/c++/14/bits/basic_string.h:4164:22: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const std::string&' {aka 'const std::__cxx11::basic_string<char>&'}
 4164 |   stoi(const string& __str, size_t* __idx = 0, int __base = 10)
      |        ~~~~~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:4432:3: note: candidate: 'int std::__cxx11::stoi(const std::wstring&, std::size_t*, int)'
 4432 |   stoi(const wstring& __str, size_t* __idx = 0, int __base = 10)
      |   ^~~~
/usr/include/c++/14/bits/basic_string.h:4432:23: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const std::wstring&' {aka 'const std::__cxx11::basic_string<wchar_t>&'}
 4432 |   stoi(const wstring& __str, size_t* __idx = 0, int __base = 10)
      |        ~~~~~~~~~~~~~~~^~~~~
a.cc:13:15: error: 'a' was not declared in this scope
   13 |     L.at(i) = a + b;
      |               ^