D.cpp

#include <bits/stdc++.h>

#define FOR(i,a,b) for(int i=(a),_b=(b); i<=_b; i++)
#define FORD(i,a,b) for(int i=(a),_b=(b); i>=_b; i--)
#define REP(i,a) for(int i=0,_a=(a); i<_a; i++)
#define EACH(it,a) for(__typeof(a.begin()) it = a.begin(); it != a.end(); ++it)

#define DEBUG(x) { cout << #x << " = "; cout << (x) << endl; }
#define PR(a,n) { cout << #a << " = "; FOR(_,1,n) cout << a[_] << ' '; cout << endl; }
#define PR0(a,n) { cout << #a << " = "; REP(_,n) cout << a[_] << ' '; cout << endl; }

#define sqr(x) ((x) * (x))
using namespace std;

int main() {
    ios :: sync_with_stdio(false);
    long long n, m, k;
    while (cin >> n >> m >> k) {
        if (k*3 < m+n) {
            cout << -1 << endl;
            continue;
        }

        int l = 0, r = m, res = 0;
        while (l <= r) {
            int mid = (l + r) >> 1;

            if (mid <= k && (k - mid) * 3 >= n + m - mid) {
                res = mid;
                l = mid + 1;
            }
            else r = mid - 1;
        }
        cout << m - res << endl;
    }
    return 0;
}