use std::collections::BTreeSet;
use std::cmp;

fn main() {
    let n: usize = {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).unwrap();
        buf.trim_end().parse().unwrap()
    };

    let a: Vec<usize> = {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).unwrap();
        let iter = buf.split_whitespace();
        iter.map(|x| x.parse().unwrap()).collect()
    };

    let mut ans = std::usize::MAX;
    let mut left = BTreeSet::new();
    let mut right = a.iter().cloned().collect::<BTreeSet<_>>();
    left.insert(a[0]);
    right.remove(&a[0]);
    for i in 1..(n - 1) {
        right.remove(&a[i]);
        match left.range(a[i]..).next() {
            Some(x) => {
                match right.range(a[i]..).next() {
                    Some(y) => {
                        ans = cmp::min(ans, x + a[i] + y);
                    },
                    None => (),
                }
            },
            None => (),
        }
        match left.range(..a[i]).next() {
            Some(x) => {
                match right.range(..a[i]).next() {
                    Some(y) => {
                        ans = cmp::min(ans, x + a[i] + y);
                    },
                    None => (),
                }
            },
            None => (),
        }
        left.insert(a[i]);
    }

    if ans == std::usize::MAX {
        println!("-1");
    } else {
        println!("{}", ans);
    }
}