#include <iostream>
#include <vector>

constexpr int INF = 1 << 30;

void solve() {
    int n;
    std::cin >> n;

    std::vector<int> dp(2, -INF);
    dp[0] = 0;

    auto ndp = dp;
    while (n--) {
        std::vector<int> ps(2);
        for (auto& p : ps) std::cin >> p;

        std::fill(ndp.begin(), ndp.end(), -INF);
        for (int i = 0; i < 2; ++i) {
            for (int j = 0; j < 2; ++j) {
                ndp[j] = std::max(ndp[j], dp[i] + ps[j] * (i ? 2 : 1));
            }
        }
        std::swap(dp, ndp);
    }

    std::cout << dp[0] << "\n";
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}