Problem p02891 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define reps(i, n) for (int i = 1; i <= (int)(n); ++i)
#define repd(i, n) for (int i = (int)(n - 1); i >= 0; --i)
#define repds(i, n) for (int i = (int)(n); i > 0; --i)
#define loop(i, x, n) for (int i = (int)(x); i < (n); ++i)
#define loops(i, x, n) for (int i = (int)(x); i <= (n); ++i)
#define loopd(i, x, n) for (int i = (int)(x); i > (n); --i)
#define loopds(i, x, n) for (int i = (int)(x); i >= (n); --i)
#define itrep(i, s) for (auto i = begin(s); i != end(s); ++i)
#define itrepd(i, s) for (auto i = --end(s); i != begin(s); --i)
#define all(f, x, ...)                                                         \
  [&](decltype(x) &whole) {                                                    \
    return (f)(begin(whole), end(whole), ##__VA_ARGS__);                       \
  }(x)
#define rall(f, x, ...)                                                        \
  [&](decltype(x) &whole) {                                                    \
    return (f)(rbegin(whole), rend(whole), ##__VA_ARGS__);                     \
  }(x)
using ll = long long;
using ld = long double;
constexpr ll inf = static_cast<ll>(1e17);
constexpr int iinf = static_cast<int>(1e9);
constexpr double dinf = 1e10;
constexpr ld ldinf = 1e17;
constexpr ll mod = static_cast<ll>(1e9 + 7);
std::ostream &endn(std::ostream &os) { return os.put(os.widen('\n')); }
template <class T> constexpr int sz(const T &a) { return (int)a.size(); }
template <class T, class... Args> constexpr T mins(T &a, Args... args) {
  return a = min<T>({a, args...});
}
template <class T, class... Args> constexpr T maxs(T &a, Args... args) {
  return a = max<T>({a, args...});
}

string s;
ll k;
void solve() {
  cin >> s >> k;

  int n = sz(s);
  char b = ' ';
  ll res = 1;
  vector<ll> vec;
  rep(i, n - 1) {
    if (s[i] == s[i + 1])
      ++res;
    else {
      vec.emplace_back(res);
      res = 1;
    }
  }
  if (res != 1)
    vec.emplace_back(res);
  ll ans = 0;
  auto m = sz(vec);
  if (m == 1)
    ans = vec[0] * k / 2;
  else if (s[0] == s[n - 1]) {
    loop(i, 1, m - 1) ans += vec[i] / 2;
    ans *= k;
    ans += (vec[0] + vec[m - 1]) / 2 * (k - 1) + vec[0] / 2 + vec[m - 1] / 2;
  } else {
    rep(i, m) ans += vec[i] / 2;
    ans *= k;
  }

  cout << ans << endn;
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  solve();
  cout << flush;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define reps(i, n) for (int i = 1; i <= (int)(n); ++i)
#define repd(i, n) for (int i = (int)(n - 1); i >= 0; --i)
#define repds(i, n) for (int i = (int)(n); i > 0; --i)
#define loop(i, x, n) for (int i = (int)(x); i < (n); ++i)
#define loops(i, x, n) for (int i = (int)(x); i <= (n); ++i)
#define loopd(i, x, n) for (int i = (int)(x); i > (n); --i)
#define loopds(i, x, n) for (int i = (int)(x); i >= (n); --i)
#define itrep(i, s) for (auto i = begin(s); i != end(s); ++i)
#define itrepd(i, s) for (auto i = --end(s); i != begin(s); --i)
#define all(f, x, ...)                                                         \
  [&](decltype(x) &whole) {                                                    \
    return (f)(begin(whole), end(whole), ##__VA_ARGS__);                       \
  }(x)
#define rall(f, x, ...)                                                        \
  [&](decltype(x) &whole) {                                                    \
    return (f)(rbegin(whole), rend(whole), ##__VA_ARGS__);                     \
  }(x)
using ll = long long;
using ld = long double;
constexpr ll inf = static_cast<ll>(1e17);
constexpr int iinf = static_cast<int>(1e9);
constexpr double dinf = 1e10;
constexpr ld ldinf = 1e17;
constexpr ll mod = static_cast<ll>(1e9 + 7);
std::ostream &endn(std::ostream &os) { return os.put(os.widen('\n')); }
template <class T> constexpr int sz(const T &a) { return (int)a.size(); }
template <class T, class... Args> constexpr T mins(T &a, Args... args) {
  return a = min<T>({a, args...});
}
template <class T, class... Args> constexpr T maxs(T &a, Args... args) {
  return a = max<T>({a, args...});
}

string s;
ll k;
void solve() {
  cin >> s >> k;

  int n = sz(s);
  char b = ' ';
  ll res = 1;
  vector<ll> vec;
  rep(i, n - 1) {
    if (s[i] == s[i + 1])
      ++res;
    else {
      vec.emplace_back(res);
      res = 1;
    }
  }
  vec.emplace_back(res);
  ll ans = 0;
  auto m = sz(vec);
  if (m == 1)
    ans = vec[0] * k / 2;
  else if (s[0] == s[n - 1]) {
    loop(i, 1, m - 1) ans += vec[i] / 2;
    ans *= k;
    ans += (vec[0] + vec[m - 1]) / 2 * (k - 1) + vec[0] / 2 + vec[m - 1] / 2;
  } else {
    rep(i, m) ans += vec[i] / 2;
    ans *= k;
  }

  cout << ans << endn;
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  solve();
  cout << flush;
  return 0;
}