A.cpp

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;

#define rep(i, a, b) for (auto i = (a); i < (b); ++i)
#define per(i, a, b) for (auto i = (b); i-- > (a); )
#define all(x) begin(x), end(x)
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) int((x).size())
#define lb(x...) lower_bound(x)
#define ub(x...) upper_bound(x)

template<class T> bool uin(T& a, const T& b) { return a > b ? a = b, 1 : 0; }
template<class T> bool uax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	ll a, b, c; cin >> a >> b >> c;
	ll d = a + c - 2 * b;
	if (d < 0) cout << abs(d) << '\n';
	else {
		if (d & 1) cout << d / 2 + 2 << '\n';
		else cout << d / 2 << '\n';
	}
}