#include <bits/stdc++.h>
using namespace std;
using ll = long long;

const ll mod = 1000000007;
const int INF = 1001001001;

int dp[100005][2]; //dp[i][j]はi番目についてj=0, 1(p[i]倒すなら0, a[i]倒すなら1)の時のスコア

int main(){
    int n; cin >> n;
    vector<int> a(n), p(n);
    for(int i = 0; i < n; i++) cin >> p[i] >> a[i];
    dp[0][0] = p[0];
    dp[0][1] = a[0];
    for(int i = 1; i < n; i++){
        dp[i][0] = max(dp[i-1][0] + p[i], dp[i-1][1] + 2 * p[i]);
        dp[i][1] = max(dp[i-1][0] + a[i], dp[i-1][1] + 2 * a[i]);
    }
    ll ans = max(dp[n-1][0], dp[n-1][1]);
    cout << ans << endl;
}