#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int m;
    cin >> m;
    int n = 2 * m;
    vector<ll> a(n);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }

    if (n % 2 == 0) {
        vector<ll> L(n, 0), R(n + 1, 0);
        for (int i = 1; i < n; i += 2) {
            L[i] = a[i - 1] - a[i];
            if (i > 1) L[i] += L[i - 2];
        }

        for (int i = n - 2; i >= 0; --i) {
            R[i] = a[i + 1] - a[i];
            if (i < n - 2) R[i] += R[i + 2];
        }
        ll ans = R[0];
        for (int i = 1; i < n; ++i) {
            ans = max(ans, L[i] + R[i + 1]);
        }
        cout << ans << "\n";
    } else {
        vector<ll> L(n, 0), R(n + 2, 0);
        for (int i = 1; i < n; i += 2) {
            L[i] = a[i - 1] - a[i];
            if (i > 1) L[i] += L[i - 2];
        }

        for (int i = n - 2; i >= 0; --i) {
            R[i] = a[i + 1] - a[i];
            if (i < n - 2) R[i] += R[i + 2];
        }

        ll ans = a[0] + R[1];
        for (int i = 1; i < n - 1; ++i) {
            ans = max(ans, L[i] + a[i] + R[i + 2]);
        }
        cout << ans << "\n";        
    }
    return 0;
}