#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("YES");}else{puts("NO");}
#define MAX_N 200005

int main() {
    int n;
    cin >> n;

    int p[n], a[n];
    rep(i,n) cin >> p[i] >> a[i];
    int dp[n+1][2];
    rep(i,n+1)rep(j,2)dp[i][j] = 0;
    dp[0][1] = -1001001001;
    rep(i,n){
        dp[i+1][0] = max(dp[i][0]+p[i], dp[i][1]+p[i]+p[i]);
        dp[i+1][1] = max(dp[i][0]+a[i], dp[i][1]+a[i]+a[i]);
    }

    int ans = dp[n][0];

    cout << ans << endl;
    return 0;
}