Problem p01523 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <bitset>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
// #include <utility>
// #include <memory>
// #include <functional>
// #include <deque>
// #include <cctype>
// #include <ctime>
// #include <numeric>
// #include <list>
// #include <iomanip>

// #if __cplusplus >= 201103L
// #include <array>
// #include <tuple>
// #include <initializer_list>
// #include <forward_list>
//
// #define cauto const auto&
// #else

// #endif

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

typedef vector<int> vint;
typedef vector<vector<int>> vvint;
typedef vector<long long> vll, vLL;
typedef vector<vector<long long>> vvll, vvLL;

#define VV(T) vector<vector<T>>

template <class T>
void initvv(vector<vector<T>> &v, int a, int b, const T &t = T()) {
  v.assign(a, vector<T>(b, t));
}

template <class F, class T> void convert(const F &f, T &t) {
  stringstream ss;
  ss << f;
  ss >> t;
}

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define reep(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) reep((i), 0, (n))
#define ALL(v) (v).begin(), (v).end()
#define PB push_back
#define F first
#define S second
#define mkp make_pair
#define RALL(v) (v).rbegin(), (v).rend()
#define DEBUG
#ifdef DEBUG
#define dump(x) cout << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
#else
#define dump(x)
#define debug(x)
#endif
#define LDcout(x, n) fixed << setprecision(n) << x

#define MOD 1000000007LL
#define EPS 1e-8
static const int INF = 1 << 24;

void mainmain() {
  int n, m;
  cin >> n >> m;
  vint v(n, 0);
  vector<pii> w(m);
  rep(i, m) {
    cin >> w[i].F >> w[i].S;
    w[i].F--;
    w[i].S--;
    reep(j, w[i].F, w[i].S + 1) { v[j]++; }
  }
  rep(i, n) {
    if (v[i] == 0) {
      cout << "Impossible" << endl;
      return;
    }
  }
  int ans = 0;
  int pos = 0;
  while (pos < n) {
    int t = 0;
    ans++;
    rep(i, n) {
      if (w[i].F <= pos && pos <= w[i].S) {
        t = max(t, w[i].S);
      }
    }
    pos = t + 1;
  }
  cout << ans << endl;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(0);
  mainmain();
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
// #include <utility>
// #include <memory>
// #include <functional>
// #include <deque>
// #include <cctype>
// #include <ctime>
// #include <numeric>
// #include <list>
// #include <iomanip>

// #if __cplusplus >= 201103L
// #include <array>
// #include <tuple>
// #include <initializer_list>
// #include <forward_list>
//
// #define cauto const auto&
// #else

// #endif

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

typedef vector<int> vint;
typedef vector<vector<int>> vvint;
typedef vector<long long> vll, vLL;
typedef vector<vector<long long>> vvll, vvLL;

#define VV(T) vector<vector<T>>

template <class T>
void initvv(vector<vector<T>> &v, int a, int b, const T &t = T()) {
  v.assign(a, vector<T>(b, t));
}

template <class F, class T> void convert(const F &f, T &t) {
  stringstream ss;
  ss << f;
  ss >> t;
}

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define reep(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) reep((i), 0, (n))
#define ALL(v) (v).begin(), (v).end()
#define PB push_back
#define F first
#define S second
#define mkp make_pair
#define RALL(v) (v).rbegin(), (v).rend()
#define DEBUG
#ifdef DEBUG
#define dump(x) cout << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
#else
#define dump(x)
#define debug(x)
#endif
#define LDcout(x, n) fixed << setprecision(n) << x

#define MOD 1000000007LL
#define EPS 1e-8
static const int INF = 1 << 24;

void mainmain() {
  int n, m;
  cin >> n >> m;
  vint v(n, 0);
  vector<pii> w(m);
  rep(i, m) {
    cin >> w[i].F >> w[i].S;
    w[i].F--;
    w[i].S--;
    reep(j, w[i].F, w[i].S + 1) { v[j]++; }
  }
  rep(i, n) {
    if (v[i] == 0) {
      cout << "Impossible" << endl;
      return;
    }
  }
  int ans = 0;
  int pos = 0;
  while (pos < n) {
    int t = 0;
    ans++;
    rep(i, m) {
      if (w[i].F <= pos && pos <= w[i].S) {
        t = max(t, w[i].S);
      }
    }
    pos = t + 1;
  }
  cout << ans << endl;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(0);
  mainmain();
}