#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)

template<class T>
inline bool chmax(T& a, T b){if(a<b){a=b;return true;} return false;}

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  int n;cin>>n;
  int p[n],a[n];
  rep(i,n)cin>>p[i]>>a[i];

  vector<vector<int>> dp(n+1,vector<int>(2,-1));
  dp[0][0]=0;
  rep(i,n){
    rep(j,2){
      if(dp[i][j]==-1)continue;
      if(j==0){
        chmax(dp[i+1][0],dp[i][j]+p[i]);
        chmax(dp[i+1][1],dp[i][j]+a[i]);
      }else{
        chmax(dp[i+1][0],dp[i][j]+p[i]+p[i]);
        chmax(dp[i+1][1],dp[i][j]+a[i]+a[i]);
      }
    }
  }
  cout<<max(dp[n][0],dp[n][1])<<endl;
}