Problem p03799 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <bits/stdc++.h>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define DEBUG(x) cout << '>' << #x << ':' << x << endl;
#define REP(i, n) for (int i = 0; i < (n); i++)
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define FORD(i, a, b) for (int i = (a); i >= (b); i--)
inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }
const int INF = 1 << 29;
typedef long long int ll;
inline int two(int n) { return 1 << n; }
inline int test(int n, int b) { return (n >> b) & 1; }
inline void set_bit(int &n, int b) { n |= two(b); }
inline void unset_bit(int &n, int b) { n &= ~two(b); }
inline int last_bit(int n) { return n & (-n); }
inline int ones(int n) {
  int res = 0;
  while (n && ++res)
    n -= n & (-n);
  return res;
}
template <class T> void chmax(T &a, const T &b) { a = max(a, b); }
template <class T> void chmin(T &a, const T &b) { a = min(a, b); }
const int MOD = 1000000007;

bool isPowerOfTwo(ll x) { return x && (!(x & (x - 1))); }
bool isPrime(int n) {
  if (n <= 1)
    return false;
  if (n <= 3)
    return true;
  if (n % 2 == 0 || n % 3 == 0)
    return false;

  for (int i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0)
      return false;

  return true;
}
int highestPowerof2(unsigned int n) {
  // Invalid input
  if (n < 1)
    return 0;

  int res = 1;
  for (int i = 0; i < 8 * sizeof(unsigned int); i++) {
    int curr = 1 << i;
    if (curr > n)
      break;

    res = curr;
  }

  return res;
}
long long binpow(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a;
    a = a * a;
    b >>= 1;
  }
  return res;
}
const int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};
const int dy[] = {0, -1, 0, 1, 1, -1, -1, 1};
/*lcm(gcd(N1, M), gcd(N2, M), ..., gcd(Nk, M)) = gcd(lcm(N1, ..., Nk), M)
gcd(lcm(N1, M), lcm(N2, M), ..., lcm(Nk, M)) = lcm(gcd(N1, ..., Nk), M).

If gcd(N1, N2) = 1, then
gcd(N1·N2, M) = gcd(N1, M)·gcd(N2, M)
lcm(N1·N2, M) = lcm(N1, M)·lcm(N2, M)/M.

lcm(M, N, P) · gcd(M, N) · gcd(N, P) · gcd(P, M) = NMP · gcd(N, M, P).
*/
////////////////////////////////////////////////////////////////////

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ll t = 1;
  while (t--) {
    ll n, m;
    cin >> n >> m;
    if (2 * n >= m)
      return cout << m / 2, 0;
    while (m > 2 * n) {
      m -= 2;
      n++;
    }
    cout << min(n, m / 2);
  }
  return 0;
}
/* Look for:
 * the exact constraints (multiple sets are too slow for n=10^6 :( )
 * special cases (n=1?)
 * 1LL<<i and not 1<<i
 * overflow (int vs ll?)
 * array bounds
 * if you have no idea just guess the appropriate well-known algo instead of
 * doing nothing :/
 */

Fixed submission:
#include <algorithm>
#include <bits/stdc++.h>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define DEBUG(x) cout << '>' << #x << ':' << x << endl;
#define REP(i, n) for (int i = 0; i < (n); i++)
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define FORD(i, a, b) for (int i = (a); i >= (b); i--)
inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }
const int INF = 1 << 29;
typedef long long int ll;
inline int two(int n) { return 1 << n; }
inline int test(int n, int b) { return (n >> b) & 1; }
inline void set_bit(int &n, int b) { n |= two(b); }
inline void unset_bit(int &n, int b) { n &= ~two(b); }
inline int last_bit(int n) { return n & (-n); }
inline int ones(int n) {
  int res = 0;
  while (n && ++res)
    n -= n & (-n);
  return res;
}
template <class T> void chmax(T &a, const T &b) { a = max(a, b); }
template <class T> void chmin(T &a, const T &b) { a = min(a, b); }
const int MOD = 1000000007;

bool isPowerOfTwo(ll x) { return x && (!(x & (x - 1))); }
bool isPrime(int n) {
  if (n <= 1)
    return false;
  if (n <= 3)
    return true;
  if (n % 2 == 0 || n % 3 == 0)
    return false;

  for (int i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0)
      return false;

  return true;
}
int highestPowerof2(unsigned int n) {
  // Invalid input
  if (n < 1)
    return 0;

  int res = 1;
  for (int i = 0; i < 8 * sizeof(unsigned int); i++) {
    int curr = 1 << i;
    if (curr > n)
      break;

    res = curr;
  }

  return res;
}
long long binpow(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a;
    a = a * a;
    b >>= 1;
  }
  return res;
}
const int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};
const int dy[] = {0, -1, 0, 1, 1, -1, -1, 1};
/*lcm(gcd(N1, M), gcd(N2, M), ..., gcd(Nk, M)) = gcd(lcm(N1, ..., Nk), M)
gcd(lcm(N1, M), lcm(N2, M), ..., lcm(Nk, M)) = lcm(gcd(N1, ..., Nk), M).

If gcd(N1, N2) = 1, then
gcd(N1·N2, M) = gcd(N1, M)·gcd(N2, M)
lcm(N1·N2, M) = lcm(N1, M)·lcm(N2, M)/M.

lcm(M, N, P) · gcd(M, N) · gcd(N, P) · gcd(P, M) = NMP · gcd(N, M, P).
*/
////////////////////////////////////////////////////////////////////

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ll t = 1;
  while (t--) {
    ll n, m;
    cin >> n >> m;
    ll res = 0;
    if (2 * n <= m) {
      res += n;
      m -= (2 * n);
      if (m >= 4)
        res += m / 4;
    } else
      res += m / 2;
    cout << res;
  }
  return 0;
}
/* Look for:
 * the exact constraints (multiple sets are too slow for n=10^6 :( )
 * special cases (n=1?)
 * 1LL<<i and not 1<<i
 * overflow (int vs ll?)
 * array bounds
 * if you have no idea just guess the appropriate well-known algo instead of
 * doing nothing :/
 */