#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using u64 = unsigned long long;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

int main() {
    int n;
    cin >> n;
    vector<int> dp1(n+1, -INF<int>), dp2(n+1, -INF<int>);
    dp1[0] = 0;
    for (int i = 0; i < n; ++i) {
        int p, a;
        scanf("%d %d", &p, &a);
        dp1[i+1] = max(dp1[i]+p, dp2[i]+p*2);
        dp2[i+1] = max(dp1[i]+a, dp2[i]+a*2);
    }
    cout << max(dp1.back(), dp2.back()) << "\n";
    return 0;
}