#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (int)(n); i ++)
#define irep(i,n) for (int i = (int)(n) - 1;i >= 0;--i)
using namespace std;
using ll = long long;
using PL = pair<ll,ll>;
using P = pair<int,int>;
constexpr int INF = 1000000000;
constexpr long long HINF = 1000000000000000;
constexpr long long MOD = 1000000007;// = 998244353;
constexpr double EPS = 1e-4;
constexpr double PI = 3.14159265358979;

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

    int N; cin >> N;
    vector<int> P(N),A(N);
    rep(i,N) cin >> P[i] >> A[i];

    vector<vector<int>> dp(N + 1,vector<int>(2,0));
    dp[0][0] = -INF;
    rep(i,N) {
        dp[i + 1][0] = max(dp[i][0] + 2*A[i],dp[i][1] + A[i]);
        dp[i + 1][1] = max(dp[i][0] + 2*P[i],dp[i][1] + P[i]);
    }
    cout << max(dp[N][0],dp[N][1]) << '\n';
    return 0;
}