use std::cmp;
use std::fs::File;
use std::io::Read;

// fn pow_speedt_with_mod(mut p: i64, mut q: i64, m: i64) -> i64 {
//     p %= m;
//     let mut r = p;
//     let mut ret: i64 = 1;
//     while q > 0 {
//         ret *= if q % 2 == 1 {r} else {1};
//         r *= r;
//         r %= m;
//         q /= 2;
//         ret %= m;
//     }
//     return ret;
// }

fn main() {
    let inputstatus = 1;
    let mut buf = String::new();

    let filename = "inputrust.txt";

    if inputstatus == 0 {
        let mut f = File::open(filename).expect("file not found");
        f.read_to_string(&mut buf)
            .expect("something went wrong reading the file");
    } else {
        std::io::stdin().read_to_string(&mut buf).unwrap();
    }

    let mut iter = buf.split_whitespace();
    let n: usize = iter.next().unwrap().parse().unwrap();
    let mut dp = vec![vec![0; n]; 2];

    for i in 0..n {
        let p: i32 = iter.next().unwrap().parse().unwrap();
        let a: i32 = iter.next().unwrap().parse().unwrap();
        if i == 0 {
            dp[0][i] = p;
            dp[1][i] = a;
        } else {
            dp[0][i] = cmp::max(dp[0][i - 1] + p, dp[1][i - 1] + 2 * p);
            dp[1][i] = cmp::max(dp[0][i - 1] + a, dp[1][i - 1] + 2 * a);
        }
    }

    println!("{}", cmp::max(dp[0][n - 1], dp[1][n - 1]));
}