Problem p03579 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#define _USE_MATH_DEFINES
#include <math.h>
#include <unordered_set>

using namespace std;

inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}

template <class T> inline T sqr(T x) { return x * x; }

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<long long int> vll;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;
typedef unsigned long long ull;

// repetition
//------------------------------------------
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define P(p) cout << (p) << endl;
#define VEC_2D(a, b) vector<vector<int>>(a, vector<int>(b, 0))
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define INF (1000000000)
#define SZ(a) int((a).size())
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define MOD 1000000007LL
#define FSP(a) cout << fixed << setprecision(a)

template <typename T> T gcd(T x, T y) {
  if (y == 0)
    return x;
  else
    return gcd(y, x % y);
}

template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template <typename T> bool is_prime(T n) {
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return n != 1;
}
map<int, int> prime_factor(int n) {
  map<int, int> res;
  for (int i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ++res[i];
      n /= i;
    }
  }
  if (n != 1)
    res[n] = 1;
  return res;
}
int extgcd(int a, int b, int &x, int &y) { //
  int d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1;
    y = 0;
  }
  return d;
}

ll mod_pow(ll x, ll n, ll mod) {
  if (n == 0)
    return 1;
  ll res = mod_pow(x * x % mod, n / 2, mod);
  if (n & 1)
    res = res * x % mod;
  return res;
}
vector<string> split(const string &str, char delim) {
  vector<string> res;
  size_t current = 0, found;
  while ((found = str.find_first_of(delim, current)) != string::npos) {
    res.push_back(string(str, current, found - current));
    current = found + 1;
  }
  res.push_back(string(str, current, str.size() - current));
  return res;
}

bool is_kadomatsu(int a, int b, int c) {
  if (a == b || a == c || b == c)
    return false;
  if (a > b && c > b)
    return true;
  if (a < b && c < b)
    return true;
  return false;
}

struct UF {
  int n;
  vi d;
  UF() {}
  UF(int n) : n(n), d(n, -1) {}
  int root(int v) {
    if (d[v] < 0)
      return v;
    return d[v] = root(d[v]);
  }
  bool same(int a, int b) { return root(a) == root(b); }
  bool unite(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (size(x) < size(y))
      swap(x, y);
    d[x] += d[y];
    d[y] = x;
    return true;
  }
  int size(int v) { return -d[root(v)]; }
};

vector<int> divisor(int n) {
  if (n == 1)
    return {};
  vi res;
  for (int i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      res.emplace_back(i);
      if (i != 1 && i != n / i)
        res.emplace_back(n / i);
    }
  }
  return res;
}

struct Bellmanford {
  int n;
  struct edge {
    int from, to, cost;
  };
  vector<edge> E;
  vi d;
  Bellmanford(int n) : n(n), d(n) { E.resize(n); }
  void add_edge(int x, int y, int cost) {
    edge e;
    e.from = x;
    e.to = y;
    e.cost = cost;
    E.push_back(e);
  }
  void shortest_path(int s) {
    rep(i, n) d[i] = INF;
    d[s] = 0;
    while (true) {
      bool update = false;
      for (auto e : E) {
        if (d[e.from] != INF && d[e.to] > d[e.from] + e.cost) {
          d[e.to] = d[e.from] + e.cost;
          update = true;
        }
      }
      if (!update)
        break;
    }
  }
};
struct Dijkstra {
  int n;
  struct edge {
    int to;
    ll cost;
  };
  vector<vector<edge>> G;
  vll d;
  Dijkstra(int n) : n(n), d(n) { G.resize(n); }
  void add_edge(int x, int y, ll cost) {
    edge e;
    e.to = y;
    e.cost = cost;
    G[x].push_back(e);
  }
  void shortest_path(int s) {
    rep(i, n) d[i] = 100000000000000000;
    d[s] = 0;
    priority_queue<pair<ll, int>, vector<pair<ll, int>>, greater<pair<ll, int>>>
        que;
    que.push(make_pair(0, s));

    while (!que.empty()) {
      pii p = que.top();
      que.pop();
      int v = p.second;
      if (d[v] < p.first)
        continue;
      for (auto e : G[v]) {
        if (d[e.to] > d[v] + e.cost) {
          d[e.to] = d[v] + e.cost;
          que.push(make_pair(d[e.to], e.to));
        }
      }
    }
  }
};

struct Segmenttree {
  int n;
  vector<pair<ll, int>> dat;
  Segmenttree() {}
  void init(ll n_) {
    n = 1;
    while (n < n_)
      n *= 2;
    dat.resize(2 * n - 1);
    rep(i, 2 * n - 1) dat[i] = pair<ll, int>(-INF, -INF);
  }
  void update(int idx, ll val) {
    idx += n - 1;
    dat[idx] = make_pair(val, -(idx - n + 1));
    while (idx > 0) {
      idx = (idx - 1) / 2;
      dat[idx] = max(dat[idx * 2 + 1], dat[idx * 2 + 2]);
    }
  }
  pair<ll, int> query(int a, int b) { return query_seg(a, b, 0, 0, n); }
  pair<ll, int> query_seg(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return pair<ll, int>(-INF, -INF);
    if (a <= l && r <= b)
      return dat[k];
    else {
      return max(query_seg(a, b, k * 2 + 1, l, (l + r) / 2),
                 query_seg(a, b, k * 2 + 2, (l + r) / 2, r));
    }
  }
};
//-------------------------------------------------------------

int color[100005];
vi g[100005];

bool dfs(int from, int now, int iro) {
  color[now] = iro;
  for (int v : g[now]) {
    if (v == from)
      continue;
    if (color[v] == iro)
      return false;
    if (!dfs(now, v, -iro))
      return false;
  }
  return true;
}
int main() {

  ll N, M;
  cin >> N >> M;
  fill(color, color + 100005, 0);

  rep(i, M) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  if (dfs(-1, 0, 1)) {
    ll b = 0, w = 0;
    rep(i, N) {
      if (color[i] == 1)
        b++;
      else
        w++;
    }
    P(b * w - M);
  } else {
    P(N * (N - 1) / 2 - M);
  }
  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#define _USE_MATH_DEFINES
#include <math.h>
#include <unordered_set>

using namespace std;

inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}

template <class T> inline T sqr(T x) { return x * x; }

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<long long int> vll;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;
typedef unsigned long long ull;

// repetition
//------------------------------------------
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define P(p) cout << (p) << endl;
#define VEC_2D(a, b) vector<vector<int>>(a, vector<int>(b, 0))
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define INF (1000000000)
#define SZ(a) int((a).size())
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define MOD 1000000007LL
#define FSP(a) cout << fixed << setprecision(a)

template <typename T> T gcd(T x, T y) {
  if (y == 0)
    return x;
  else
    return gcd(y, x % y);
}

template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template <typename T> bool is_prime(T n) {
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return n != 1;
}
map<int, int> prime_factor(int n) {
  map<int, int> res;
  for (int i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ++res[i];
      n /= i;
    }
  }
  if (n != 1)
    res[n] = 1;
  return res;
}
int extgcd(int a, int b, int &x, int &y) { //
  int d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1;
    y = 0;
  }
  return d;
}

ll mod_pow(ll x, ll n, ll mod) {
  if (n == 0)
    return 1;
  ll res = mod_pow(x * x % mod, n / 2, mod);
  if (n & 1)
    res = res * x % mod;
  return res;
}
vector<string> split(const string &str, char delim) {
  vector<string> res;
  size_t current = 0, found;
  while ((found = str.find_first_of(delim, current)) != string::npos) {
    res.push_back(string(str, current, found - current));
    current = found + 1;
  }
  res.push_back(string(str, current, str.size() - current));
  return res;
}

bool is_kadomatsu(int a, int b, int c) {
  if (a == b || a == c || b == c)
    return false;
  if (a > b && c > b)
    return true;
  if (a < b && c < b)
    return true;
  return false;
}

struct UF {
  int n;
  vi d;
  UF() {}
  UF(int n) : n(n), d(n, -1) {}
  int root(int v) {
    if (d[v] < 0)
      return v;
    return d[v] = root(d[v]);
  }
  bool same(int a, int b) { return root(a) == root(b); }
  bool unite(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (size(x) < size(y))
      swap(x, y);
    d[x] += d[y];
    d[y] = x;
    return true;
  }
  int size(int v) { return -d[root(v)]; }
};

vector<int> divisor(int n) {
  if (n == 1)
    return {};
  vi res;
  for (int i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      res.emplace_back(i);
      if (i != 1 && i != n / i)
        res.emplace_back(n / i);
    }
  }
  return res;
}

struct Bellmanford {
  int n;
  struct edge {
    int from, to, cost;
  };
  vector<edge> E;
  vi d;
  Bellmanford(int n) : n(n), d(n) { E.resize(n); }
  void add_edge(int x, int y, int cost) {
    edge e;
    e.from = x;
    e.to = y;
    e.cost = cost;
    E.push_back(e);
  }
  void shortest_path(int s) {
    rep(i, n) d[i] = INF;
    d[s] = 0;
    while (true) {
      bool update = false;
      for (auto e : E) {
        if (d[e.from] != INF && d[e.to] > d[e.from] + e.cost) {
          d[e.to] = d[e.from] + e.cost;
          update = true;
        }
      }
      if (!update)
        break;
    }
  }
};
struct Dijkstra {
  int n;
  struct edge {
    int to;
    ll cost;
  };
  vector<vector<edge>> G;
  vll d;
  Dijkstra(int n) : n(n), d(n) { G.resize(n); }
  void add_edge(int x, int y, ll cost) {
    edge e;
    e.to = y;
    e.cost = cost;
    G[x].push_back(e);
  }
  void shortest_path(int s) {
    rep(i, n) d[i] = 100000000000000000;
    d[s] = 0;
    priority_queue<pair<ll, int>, vector<pair<ll, int>>, greater<pair<ll, int>>>
        que;
    que.push(make_pair(0, s));

    while (!que.empty()) {
      pii p = que.top();
      que.pop();
      int v = p.second;
      if (d[v] < p.first)
        continue;
      for (auto e : G[v]) {
        if (d[e.to] > d[v] + e.cost) {
          d[e.to] = d[v] + e.cost;
          que.push(make_pair(d[e.to], e.to));
        }
      }
    }
  }
};

struct Segmenttree {
  int n;
  vector<pair<ll, int>> dat;
  Segmenttree() {}
  void init(ll n_) {
    n = 1;
    while (n < n_)
      n *= 2;
    dat.resize(2 * n - 1);
    rep(i, 2 * n - 1) dat[i] = pair<ll, int>(-INF, -INF);
  }
  void update(int idx, ll val) {
    idx += n - 1;
    dat[idx] = make_pair(val, -(idx - n + 1));
    while (idx > 0) {
      idx = (idx - 1) / 2;
      dat[idx] = max(dat[idx * 2 + 1], dat[idx * 2 + 2]);
    }
  }
  pair<ll, int> query(int a, int b) { return query_seg(a, b, 0, 0, n); }
  pair<ll, int> query_seg(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return pair<ll, int>(-INF, -INF);
    if (a <= l && r <= b)
      return dat[k];
    else {
      return max(query_seg(a, b, k * 2 + 1, l, (l + r) / 2),
                 query_seg(a, b, k * 2 + 2, (l + r) / 2, r));
    }
  }
};
//-------------------------------------------------------------

int color[100005];
vi g[100005];

bool dfs(int from, int now, int iro) {
  color[now] = iro;
  for (int v : g[now]) {
    if (v == from)
      continue;
    if (color[v] == iro)
      return false;
    if (color[v] == 0 && !dfs(now, v, -iro))
      return false;
  }
  return true;
}
int main() {

  ll N, M;
  cin >> N >> M;
  fill(color, color + 100005, 0);

  rep(i, M) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  if (dfs(-1, 0, 1)) {
    ll b = 0, w = 0;
    rep(i, N) {
      if (color[i] == 1)
        b++;
      else
        w++;
    }
    P(b * w - M);
  } else {
    P(N * (N - 1) / 2 - M);
  }
  return 0;
}