Language: C++

#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef long double ld;
typedef vector<ll> vl;
typedef vector<string> vs;
typedef vector<char> vc;
typedef queue<ll> ql;
typedef deque<ll> dql;
typedef priority_queue<ll/*, vl, greater<ll>*/> pql; //降順(/*昇順*/)
typedef set<ll> sl;
typedef pair<ll, ll> pl;
typedef vector<vl> vvl;
typedef vector<pl> vpl;
#define rep(i, n) for(ll i = 0; i < ll(n); i++)
#define rep2(i, n) for(ll i = 1; i <= ll(n); i++)
//#define rep(i, k, n) for(ll i = k-1; i < ll(n); i++)
//#define rep2(i, k, n) for(ll i = k; i <= ll(n); i++)
#define all(v) (v).begin(), (v).end()
bool chmin(ll &a, ll b) {if(b < a) {a = b; return 1;} return 0;}
bool chmax(ll &a, ll b) {if(b > a) {a = b; return 1;} return 0;}
const ll INF = 1LL << 60;
const ll MOD = 1e9 + 7;
//const ll MOD = 998244353;
const ll MAX = 1e9;
const char newl = '\n';

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  string s;
  cin >> s;
  ll ans=0;
  rep(i, n) if(i+s[i] & 1) ans++;
  cout << min(ans, s.size()-ans) << newl;
  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:35:10: error: 'n' was not declared in this scope
   35 |   rep(i, n) if(i+s[i] & 1) ans++;
      |          ^
a.cc:15:40: note: in definition of macro 'rep'
   15 | #define rep(i, n) for(ll i = 0; i < ll(n); i++)
      |                                        ^
a.cc:36:14: error: no matching function for call to 'min(ll&, long long unsigned int)'
   36 |   cout << min(ans, s.size()-ans) << newl;
      |           ~~~^~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:233:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::min(const _Tp&, const _Tp&)'
  233 |     min(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:233:5: note:   template argument deduction/substitution failed:
a.cc:36:14: note:   deduced conflicting types for parameter 'const _Tp' ('long long int' and 'long long unsigned int')
   36 |   cout << min(ans, s.size()-ans) << newl;
      |           ~~~^~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::min(const _Tp&, const _Tp&, _Compare)'
  281 |     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: 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:5686:5: note: candidate: 'template<class _Tp> constexpr _Tp std::min(initializer_list<_Tp>)'
 5686 |     min(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5686:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5696:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::min(initializer_list<_Tp>, _Compare)'
 5696 |     min(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note:   template argument deduction/substitution failed:
a.cc:36:14: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   36 |   cout << min(ans, s.size()-ans) << newl;
      |           ~~~^~~~~~~~~~~~~~~~~~~