use std::{
    collections::{BTreeSet, HashMap},
    hash::Hash,
};

fn main() {
    proconio::input! {
        n: usize,
        mut v: [i64; n],
    }
    v.reverse();
    let mut value = 0;
    let mut i = 0;
    while i < n {
        let v2 = *v.get(i + 1).unwrap_or(&0);
        if v[i] >= v2 {
            value += v[i];
            i += 2;
        } else {
            value += v2;
            i += 3;
        }
    }
    println!("{value}");
}