#include <cassert>
#include <iostream>
#include <mutex>
#include <thread>

struct Account {
    std::mutex mutex;
    int balance = 1000;
};

void transfer_one(Account& from, Account& to) {
    if (&from == &to) return;
    std::scoped_lock lock(from.mutex, to.mutex);
    assert(from.balance > 0);
    --from.balance;
    ++to.balance;
}

int main() {
    Account a;
    Account b;
    std::thread worker([&] {
        for (int i = 0; i < 100; ++i) transfer_one(a, b);
    });
    for (int i = 0; i < 100; ++i) transfer_one(b, a);
    worker.join();
    assert(a.balance == 1000 && b.balance == 1000);
    transfer_one(a, a);
    assert(a.balance + b.balance == 2000);
    std::cout << a.balance << ' ' << b.balance << '\n';
}