Problem p02661 (C++) — buggy submission, verdict: Runtime Error

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

#define rep(i, n) for (ll i = 0; i < (n); ++i)
using ll = int_fast64_t;
using pll = pair<ll, ll>;
constexpr ll INF = 1LL << 60;
constexpr ll MOD = 1e9 + 7;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
#if loc || local
void dump() { cerr << endl; }
template <class T, class... Ts> void dump(T &&h, Ts &&...t) {
  cerr << h << ", ";
  dump(forward<Ts>(t)...);
}
#else
void dump() {}
template <class T, class... Ts> void dump(T &&h, Ts &&...t) {}
#endif
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &elemnt : v)
    is >> elemnt;
  return is;
}
template <class T, class U> istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> ostream &operator<<(ostream &os, vector<T> const &v) {
  for (auto const &vi : v)
    os << vi << " ";
  return os;
}
template <class T, class U>
ostream &operator<<(ostream &os, pair<T, U> const &p) {
  os << p.first << "," << p.second;
  return os;
}
template <class T> vector<T> vec(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto vec(size_t a, Ts... ts) {
  return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...));
}

signed main() {

  ll n;
  cin >> n;
  vector<ll> a(n), b(n);
  rep(i, n) cin >> a[i] >> b[i];

  sort(begin(a), end(a));
  sort(begin(b), end(b));
  if (n & 1) {
    int m = n / 2;
    cout << b[m] - a[m] + 1 << endl;
  } else {
    int m = n / 2;
    double mn = (a[m] + a[m - 1]) / 2.0;
    double mx = (b[m] + b[m - 1]) / 2.0;
    double ansd = (mx - mn) / 0.5;
    ll ans = ansd + 1;
    cout << ans << endl;
  }
}

Error: 1

Fixed submission:

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

#define rep(i, n) for (ll i = 0; i < (n); ++i)
using ll = int_fast64_t;
using pll = pair<ll, ll>;
constexpr ll INF = 1LL << 60;
constexpr ll MOD = 1e9 + 7;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
#if loc || local
void dump() { cerr << endl; }
template <class T, class... Ts> void dump(T &&h, Ts &&...t) {
  cerr << h << ", ";
  dump(forward<Ts>(t)...);
}
#else
void dump() {}
template <class T, class... Ts> void dump(T &&h, Ts &&...t) {}
#endif
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &elemnt : v)
    is >> elemnt;
  return is;
}
template <class T, class U> istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> ostream &operator<<(ostream &os, vector<T> const &v) {
  for (auto const &vi : v)
    os << vi << " ";
  return os;
}
template <class T, class U>
ostream &operator<<(ostream &os, pair<T, U> const &p) {
  os << p.first << "," << p.second;
  return os;
}
template <class T> vector<T> vec(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto vec(size_t a, Ts... ts) {
  return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...));
}

signed main() {

  ll n;
  cin >> n;
  vector<ll> a(n), b(n);
  rep(i, n) cin >> a[i] >> b[i];

  sort(begin(a), end(a));
  sort(begin(b), end(b));
  if (n & 1) {
    int m = n / 2;
    cout << b[m] - a[m] + 1 << endl;
  } else {
    return 1;
  }
}