#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <math.h>
#include <iomanip>
#include <limits>
#include <list>
#include <queue>
#include <tuple>
#include <map>
#include <stack>
#include <set>
using namespace std;
#define MOD (long long int)(1e9+7)
#define ll long long int
#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 REP(i,n) for(int i=n-1; i>=0; i--)
#define REPS(i,n) for(int i=n; i>0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a)<(b)) ? (b) : (a))
#define chmin(a, b) a = (((a)>(b)) ? (b) : (a))
#define all(v) v.begin(), v.end()

const int N = (int)3e5;

ll mpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = mpow(a,b/2); return memo*memo%MOD;}
  else return mpow(a,b-1) * a % MOD;
}
ll lpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = lpow(a,b/2); return memo*memo;}
  else return lpow(a,b-1) * a;
}
ll gcd(ll a, ll b){
  if(b==0) return a;
  else return gcd(b, a%b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n){
  if(kaijo_memo.size() > n) return kaijo_memo[n];
  if(kaijo_memo.size() == 0) kaijo_memo.push_back(1);
  while(kaijo_memo.size() <= n) kaijo_memo.push_back(kaijo_memo[kaijo_memo.size()-1] * kaijo_memo.size() % MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n){
  if(gyaku_kaijo_memo.size() > n) return gyaku_kaijo_memo[n];
  if(gyaku_kaijo_memo.size() == 0) gyaku_kaijo_memo.push_back(1);
  while(gyaku_kaijo_memo.size() <= n) gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size()-1] * mpow(gyaku_kaijo_memo.size(), MOD-2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r){
  if(n == r) return 1;//0個の丸と-1個の棒みたいな時に時に効く？不安.
  if(n < r || r < 0) return 0;
  ll ret = 1;
  ret *= kaijo(n); ret %= MOD;
  ret *= gyaku_kaijo(r); ret %= MOD;
  ret *= gyaku_kaijo(n-r); ret %= MOD;
  return ret;
}

vector<ll> R[N],B[N];
bool checked[N],exist[N];
ll dp[N][3][2];

void saiki(int now){
  if(checked[now]) return;
  checked[now] = true;
  vector<int> child;
  rep(i,R[now].size()){
    int next = R[now][i];
    if(exist[next] && !checked[next]){
      child.push_back(next);
    }
  }
  rep(i,child.size()){
    saiki(child[i]);
  }

  ll sum = 0;
  ll most = -INF*(ll)10;
  ll second = -INF*(ll)10;
  ll twoMost = -INF*(ll)10;
  int boo = B[now].size()%2;
  rep(i,child.size()){
    int next = child[i];
    int b;
    if(dp[next][0][0] < INF-30){
      b = 0;
    }else{
      b = 1;
      boo++;
      boo%=2;
    }
    sum += dp[next][0][b] + b;
    ll score = dp[next][0][b] - dp[next][1][1-b] + b*2-1;
    if(most <= score){
      second = most;
      most = score;
    }else if(second <= score){
      second = score;
    }
    ll twoScore = dp[next][0][b] - dp[next][2][b];
    chmax(twoMost, twoScore);
  }

  chmin(dp[now][0][boo],sum);

  chmin(dp[now][1][1-boo],sum - most);

  chmin(dp[now][2][boo],min(sum - most - second, sum - twoMost));
  if(B[now].size()%2 == 1){
    chmin(dp[now][2][boo],dp[now][1][1-boo]);
  }

  //cout<<now<<" "<<dp[now][0][boo]<<" "<<dp[now][1][1-boo]<<" "<<dp[now][2][boo]
  //<<" "<<sum<<endl;
}

bool sakujo(int now){
  if(checked[now]) return false;
  checked[now] = true;
  int count = 0;
  rep(i,R[now].size()){
    if(sakujo(R[now][i])){
      count++;
    }
  }
  if(count == 0){//削除対象かも？
    if(B[now].size()%2 == 1){//葉である
      exist[now] = true;
      dp[now][0][1] = 0;
      dp[now][1][0] = 0;
    }else{//葉じゃない, 削除
      exist[now] = false;
      return false;
    }
  }else{
    exist[now] = true;
  }
  return true;
}

int main(void){
  int a,b;cin>>a>>b;
  cout<<max(0,a-b*2)<<endl;
  return 0;
}


Measured memory_bytes: 15100