B. Lottery

#include <iostream>

using namespace std;

long long L, R, X, K;

int main()
{
    cin >> L >> R >> X >> K;
    long long ans = 0;
    for (long long i = 1LL << 62; i; i >>= 1)
    {
        if ((L & i) == (R & i)) continue;
        long long c = (R & ((1LL << 62) - i));
        if ((L & i) == (X & i))
            if ((c - L) >= K)
            {
                R = c - 1;
            }
            else
            {
                K -= (c - L);
                L = c;
            }
        else
            if ((R - c + 1) >= K)
            {
                L = c;
            }
            else
            {
                K -= (R - c + 1);
                R = c - 1;
            }
    }
    cout << L << endl;
    return 0;
}