/////////////////////////////////////////////////////////////////////////
// ------------------------------------------------------------------- //
// | Author: Fananhkhoi | Luong Van Chanh High School for the gifted | //
// ------------------------------------------------------------------- //
/////////////////////////////////////////////////////////////////////////
/* -------------------[ TALK LESS & DO MORE ]------------------- */
// #pragma GCC target("popcnt,lzcnt,bmi,bmi2,abm")
#include <bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define getBit(x, k) (((x) >> (k)) & 1)
#define MASK(i) (1LL << (i))
#define int long long
//#define ll long long
#define MOD 1000000007
#define el "\n"
#define vi vector<int>
#define pii pair<int,int>
#define vii vector<pii>
#define FOU(i, l, r) for (int i = l; i <= r; i ++)
#define FOD(i, r, l) for (int i = r; i >= l; i --)
#define trav(a, x) for (auto &a : x )
#define rep(i, a, b) for (int i = a; i < (b); i ++)
#define gcd __gcd
#define fi first
#define se second
#define MP make_pair
#define pb push_back
#define pf push_front
#define all(v) v.begin(), v.end()
#define rev(v) reverse(all(v))
#define srt(v) sort(all(v))
#define grtsrt(v) sort(all(v), greater<int>())
#define sz(x) (int) (x).size()
#define maxHeap priority_queue<int>
#define minHeap priority_queue<int, vi, greater<int>>
#define Fix(a, b) cout << fixed << setprecision((b)) << (a)
int dx[] = {};
int dy[] = {};
template<class _, class __>
bool minimize(_ &x, const __ y){
if(x > y){
x = y;
return true;
} else return false;
}
template<class _, class __>
bool maximize(_ &x, const __ y){
if(x < y){
x = y;
return true;
} else return false;
}
inline int max(int a, int b){
return (a > b) ? a : b;
}
inline int min(int a, int b){
return (a < b) ? a : b;
}
/* --------------------[ MAIN CODE ]--------------------- */
const int MaxN = 1e6 + 50;
const int oo = 2e18;
int n, k;
void inp(){
cin >> n >> k;
}
void sol(){
if (k >= 2 * n){
cout << "0";
}
else{
if (k > n){
int x = k - n;
int tmp = (n + x - 1) / 2;
cout << tmp - x + 1;
}
else{
int x = 1, y = k - 1;
int tmp = (x + y - 1) / 2;
cout << tmp;
}
}
}
int32_t main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//freopen("PHANANHKHOI.INP", "r", stdin);
//freopen("PHANANHKHOI.OUT", "w", stdout);
int t = 1;
bool multitest = 0;
if (multitest){
cin >> t;
}
while(t--){
inp();
sol();
}
return 0;
}
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