#include <bits/stdc++.h>

using namespace std;

using i64 = long long;

int main() {
	cin.tie(0)->sync_with_stdio(0);
	i64 a, b, c;
	cin >> a >> b >> c;
	a = b - a;
	b = c - b;
	i64 ans = 0;
	if (b > a) {
		i64 d = (b - a) / 2;
		ans += d;
		a += d;
		b -= d;
	}
	if (b > a) {
		b--;
		a++;
		ans++;
	}
	ans += max(a - b, 0ll);
	cout << ans << endl;
}