Problem:
Score: 500 points

Problem Statement

Given is a permutation P of \{1, 2, \ldots, N\}.

For a pair (L, R) (1 \le L \lt R \le N), let X_{L, R} be the second largest value among P_L, P_{L+1}, \ldots, P_R.

Find \displaystyle \sum_{L=1}^{N-1} \sum_{R=L+1}^{N} X_{L,R}.

Constraints

2 \le N \le 10^5

1 \le P_i \le N

P_i \neq P_j (i \neq j)

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
P_1 P_2 \ldots P_N

Output

Print \displaystyle \sum_{L=1}^{N-1} \sum_{R=L+1}^{N} X_{L,R}.

Sample Input 1

3
2 3 1

Sample Output 1

5

X_{1, 2} = 2, X_{1, 3} = 2, and X_{2, 3} = 1, so the sum is 2 + 2 + 1 = 5.

Sample Input 2

5
1 2 3 4 5

Sample Output 2

30

Sample Input 3

8
8 2 7 3 4 5 6 1

Sample Output 3

136

Solution in Rust (1.42.0) (judge verdict: Accepted)

pub trait IterScan: Sized {
    type Output;
    fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output>;
}
pub trait MarkedIterScan: Sized {
    type Output;
    fn mscan<'a, I: Iterator<Item = &'a str>>(self, iter: &mut I) -> Option<Self::Output>;
}
#[derive(Debug)]
pub struct Scanner<'a> {
    iter: std::str::SplitAsciiWhitespace<'a>,
}
impl<'a> Scanner<'a> {
    #[inline]
    pub fn new(s: &'a str) -> Self {
        let iter = s.split_ascii_whitespace();
        Self { iter }
    }
    #[inline]
    pub fn scan<T: IterScan>(&mut self) -> <T as IterScan>::Output {
        T::scan(&mut self.iter).unwrap()
    }
    #[inline]
    pub fn mscan<T: MarkedIterScan>(&mut self, marker: T) -> <T as MarkedIterScan>::Output {
        marker.mscan(&mut self.iter).unwrap()
    }
    #[inline]
    pub fn scan_vec<T: IterScan>(&mut self, size: usize) -> Vec<<T as IterScan>::Output> {
        (0..size)
            .map(|_| T::scan(&mut self.iter).unwrap())
            .collect()
    }
    #[inline]
    pub fn scan_chars(&mut self) -> Vec<char> {
        self.iter.next().unwrap().chars().collect::<Vec<char>>()
    }
}
mod scanner_impls {
    use super::*;
    macro_rules !iter_scan_impls {($($t :ty ) *) =>{$(impl IterScan for $t {type Output =Self ;#[inline ] fn scan <'a ,I :Iterator <Item =&'a str >>(iter :&mut I ) ->Option <Self >{iter .next () ?.parse ::<$t >() .ok () } } ) *} ;}
    iter_scan_impls !(char u8 u16 u32 u64 usize i8 i16 i32 i64 isize f32 f64 u128 i128 String ) ;
    macro_rules !iter_scan_tuple_impl {($($T :ident ) *) =>{impl <$($T :IterScan ) ,*>IterScan for ($($T ,) *) {type Output =($(<$T as IterScan >::Output ,) *) ;#[inline ] fn scan <'a ,It :Iterator <Item =&'a str >>(_iter :&mut It ) ->Option <Self ::Output >{Some (($($T ::scan (_iter ) ?,) *) ) } } } ;}
    iter_scan_tuple_impl!();
    iter_scan_tuple_impl!(A);
    iter_scan_tuple_impl !(A B ) ;
    iter_scan_tuple_impl !(A B C ) ;
    iter_scan_tuple_impl !(A B C D ) ;
    iter_scan_tuple_impl !(A B C D E ) ;
    iter_scan_tuple_impl !(A B C D E F ) ;
    iter_scan_tuple_impl !(A B C D E F G ) ;
    iter_scan_tuple_impl !(A B C D E F G H ) ;
    iter_scan_tuple_impl !(A B C D E F G H I ) ;
    iter_scan_tuple_impl !(A B C D E F G H I J ) ;
    iter_scan_tuple_impl !(A B C D E F G H I J K ) ;
    pub struct ScannerIter<'a, 'b, T> {
        inner: &'b mut Scanner<'a>,
        _marker: std::marker::PhantomData<fn() -> fn() -> T>,
    }
    impl<'a, 'b, T: IterScan> Iterator for ScannerIter<'a, 'b, T> {
        type Item = <T as IterScan>::Output;
        fn next(&mut self) -> Option<Self::Item> {
            T::scan(&mut self.inner.iter)
        }
    }
    impl<'a> Scanner<'a> {
        #[inline]
        pub fn iter<'b, T: IterScan>(&'b mut self) -> ScannerIter<'a, 'b, T> {
            ScannerIter {
                inner: self,
                _marker: std::marker::PhantomData,
            }
        }
    }
}
mod marker {
    use super::*;
    pub struct Usize1;
    impl IterScan for Usize1 {
        type Output = usize;
        #[inline]
        fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output> {
            usize::scan(iter).map(|x| x.wrapping_sub(1))
        }
    }
    pub struct Isize1;
    impl IterScan for Isize1 {
        type Output = isize;
        #[inline]
        fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output> {
            isize::scan(iter).map(|x| x.wrapping_sub(1))
        }
    }
}
#[macro_export]
macro_rules !min {($e :expr ) =>{$e } ;($e :expr ,$($es :expr ) ,+) =>{std ::cmp ::min ($e ,min !($($es ) ,+) ) } ;}
#[macro_export]
macro_rules !chmin {($dst :expr ,$($src :expr ) ,+) =>{{let x =std ::cmp ::min ($dst ,min !($($src ) ,+) ) ;$dst =x ;} } ;}
#[macro_export]
macro_rules !max {($e :expr ) =>{$e } ;($e :expr ,$($es :expr ) ,+) =>{std ::cmp ::max ($e ,max !($($es ) ,+) ) } ;}
#[macro_export]
macro_rules !chmax {($dst :expr ,$($src :expr ) ,+) =>{{let x =std ::cmp ::max ($dst ,max !($($src ) ,+) ) ;$dst =x ;} } ;}
/// binary operaion: $T \circ T \to T$
pub trait Magma {
    /// type of operands: $T$
    type T: Clone + PartialEq;
    /// binary operaion: $\circ$
    fn operate(&self, x: &Self::T, y: &Self::T) -> Self::T;
}
/// $\forall a,\forall b,\forall c \in T, (a \circ b) \circ c = a \circ (b \circ c)$
pub trait Associative {}
/// associative binary operation
pub trait SemiGroup: Magma + Associative {}
impl<S: Magma + Associative> SemiGroup for S {}
/// $\exists e \in T, \forall a \in T, e \circ a = a \circ e = e$
pub trait Unital: Magma {
    /// identity element: $e$
    fn unit(&self) -> Self::T;
}
/// associative binary operation and an identity element
pub trait Monoid: SemiGroup + Unital {
    /// binary exponentiation: $x^n = x\circ\ddots\circ x$
    fn pow(&self, x: Self::T, n: usize) -> Self::T {
        let mut n = n;
        let mut res = self.unit();
        let mut base = x;
        while n > 0 {
            if n & 1 == 1 {
                res = self.operate(&res, &base);
            }
            base = self.operate(&base, &base);
            n = n >> 1;
        }
        res
    }
}
impl<M: SemiGroup + Unital> Monoid for M {}
/// $\exists e \in T, \forall a \in T, \exists b,c \in T, b \circ a = a \circ c = e$
pub trait Invertible: Magma {
    /// $a$ where $a \circ x = e$
    fn inverse(&self, x: &Self::T) -> Self::T;
}
/// short cut of right inverse binary operation
pub trait RightInvertibleMagma: Magma {
    /// right inverse binary operation: $\cdot \circ (\cdot^{-1})$
    fn rinv_operation(&self, x: &Self::T, y: &Self::T) -> Self::T;
}
/// associative binary operation and an identity element and inverse elements
pub trait Group: Monoid + Invertible {}
impl<G: Monoid + Invertible> Group for G {}
/// $\forall a,\forall b \in T, a \circ b = b \circ a$
pub trait Commutative {}
/// commutative monoid
pub trait AbelianMonoid: Monoid + Commutative {}
impl<M: Monoid + Commutative> AbelianMonoid for M {}
/// commutative group
pub trait AbelianGroup: Group + Commutative {}
impl<G: Group + Commutative> AbelianGroup for G {}
/// $\forall a \in T, a \circ a = a$
pub trait Idempotent {}
/// idempotent monoid
pub trait IdempotentMonoid: Monoid + Idempotent {}
impl<M: Monoid + Idempotent> IdempotentMonoid for M {}
/// $+$
#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct AdditiveOperation<T: Copy + PartialEq + AdditiveIdentity> {
    _marker: std::marker::PhantomData<fn() -> T>,
}
pub trait AdditiveIdentity: Sized + std::ops::Add<Output = Self> {
    fn zero() -> Self;
}
#[macro_use]
mod additive_operation_impl {
    use super::*;
    #[macro_export(local_inner_macros)]
    macro_rules !impl_additive_identity {([$($wh :tt ) *] ,$t :ty ,$zero :expr ) =>{impl <$($wh ) *>AdditiveIdentity for $t {#[inline ] fn zero () ->Self {$zero } } } ;($t :ty ,$zero :expr ) =>{impl AdditiveIdentity for $t {#[inline ] fn zero () ->Self {$zero } } } ;}
    impl_additive_identity!(usize, 0usize);
    impl_additive_identity!(u8, 0u8);
    impl_additive_identity!(u16, 0u16);
    impl_additive_identity!(u32, 0u32);
    impl_additive_identity!(u64, 0u64);
    impl_additive_identity!(isize, 0isize);
    impl_additive_identity!(i8, 0i8);
    impl_additive_identity!(i16, 0i16);
    impl_additive_identity!(i32, 0i32);
    impl_additive_identity!(i64, 0i64);
    impl_additive_identity!(f32, 0.0f32);
    impl_additive_identity!(f64, 0.0f64);
    impl<T: Copy + PartialEq + AdditiveIdentity> AdditiveOperation<T> {
        pub fn new() -> Self {
            Self {
                _marker: std::marker::PhantomData,
            }
        }
    }
    impl<T: Copy + PartialEq + AdditiveIdentity> Magma for AdditiveOperation<T> {
        type T = T;
        #[inline]
        fn operate(&self, x: &Self::T, y: &Self::T) -> Self::T {
            *x + *y
        }
    }
    impl<T: Copy + PartialEq + AdditiveIdentity> Unital for AdditiveOperation<T> {
        #[inline]
        fn unit(&self) -> Self::T {
            AdditiveIdentity::zero()
        }
    }
    impl<T: Copy + PartialEq + AdditiveIdentity> Associative for AdditiveOperation<T> {}
    impl<T: Copy + PartialEq + AdditiveIdentity> Commutative for AdditiveOperation<T> {}
    impl<T: Copy + PartialEq + AdditiveIdentity + std::ops::Neg<Output = T>> Invertible
        for AdditiveOperation<T>
    {
        #[inline]
        fn inverse(&self, x: &Self::T) -> Self::T {
            -*x
        }
    }
    impl<T: Copy + PartialEq + AdditiveIdentity + std::ops::Sub<Output = T>> RightInvertibleMagma
        for AdditiveOperation<T>
    {
        #[inline]
        fn rinv_operation(&self, x: &Self::T, y: &Self::T) -> Self::T {
            *x - *y
        }
    }
}
#[derive(Clone, Debug)]
pub struct BinaryIndexedTree<M: Monoid> {
    n: usize,
    bit: Vec<M::T>,
    m: M,
}
impl<M: Monoid> BinaryIndexedTree<M> {
    #[inline]
    pub fn new(n: usize, m: M) -> Self {
        let bit = vec![m.unit(); n + 1];
        Self { n, bit, m }
    }
    #[inline]
    /// fold [0, k)
    pub fn accumulate0(&self, mut k: usize) -> M::T {
        debug_assert!(k <= self.n);
        let mut res = self.m.unit();
        while k > 0 {
            res = self.m.operate(&res, &self.bit[k]);
            k -= k & !k + 1;
        }
        res
    }
    #[inline]
    /// fold [0, k]
    pub fn accumulate(&self, k: usize) -> M::T {
        self.accumulate0(k + 1)
    }
    #[inline]
    pub fn update(&mut self, k: usize, x: M::T) {
        debug_assert!(k < self.n);
        let mut k = k + 1;
        while k <= self.n {
            self.bit[k] = self.m.operate(&self.bit[k], &x);
            k += k & !k + 1;
        }
    }
}
impl<G: Group> BinaryIndexedTree<G> {
    #[inline]
    pub fn fold(&self, l: usize, r: usize) -> G::T {
        debug_assert!(l < self.n && 0 < r && r <= self.n);
        self.m
            .operate(&self.m.inverse(&self.accumulate0(l)), &self.accumulate0(r))
    }
    #[inline]
    pub fn get(&self, k: usize) -> G::T {
        self.fold(k, k + 1)
    }
    #[inline]
    pub fn set(&mut self, k: usize, x: G::T) {
        let y = self.m.inverse(&self.get(k));
        self.update(k, self.m.operate(&y, &x));
    }
}
impl<M: Monoid> BinaryIndexedTree<M>
where
    M::T: Ord,
{
    #[inline]
    pub fn lower_bound(&self, x: M::T) -> usize {
        let n = self.n;
        let mut acc = self.m.unit();
        let mut pos = 0;
        let mut k = 1 << format!("{:b}", n).len();
        while k > 0 {
            if k + pos <= n && self.m.operate(&acc, &self.bit[k + pos]) < x {
                pos += k;
                acc = self.m.operate(&acc, &self.bit[pos]);
            }
            k >>= 1;
        }
        pos
    }
}

fn main() {
    #[allow(unused_imports)]
    use std::io::{Read as _, Write as _};
    let __out = std::io::stdout();
    let mut __in_buf = String::new();
    std::io::stdin().read_to_string(&mut __in_buf).unwrap();
    let mut __scanner = Scanner::new(&__in_buf);
    #[allow(unused_macros)]
    macro_rules !scan {() =>{scan !(usize ) } ;(($($t :tt ) ,*) ) =>{($(scan !($t ) ) ,*) } ;([$t :tt ;$len :expr ] ) =>{(0 ..$len ) .map (|_ |scan !($t ) ) .collect ::<Vec <_ >>() } ;({$t :tt =>$f :expr } ) =>{$f (scan !($t ) ) } ;(chars ) =>{__scanner .scan_chars () } ;($t :ty ) =>{__scanner .scan ::<$t >() } ;}
    let mut __out = std::io::BufWriter::new(__out.lock());
    #[allow(unused_macros)]
    macro_rules !print {($($arg :tt ) *) =>(::std ::write !(__out ,$($arg ) *) .unwrap () ) }
    #[allow(unused_macros)]
    macro_rules !println {($($arg :tt ) *) =>(::std ::writeln !(__out ,$($arg ) *) .unwrap () ) }
    #[allow(unused_macros)]
    macro_rules! echo {
        ($iter :expr ) => {
            echo!($iter, "\n")
        };
        ($iter :expr ,$sep :expr ) => {
            let mut iter = $iter;
            if let Some(item) = iter.next() {
                print!("{}", item);
            }
            for item in iter {
                print!("{}{}", $sep, item);
            }
            println!();
        };
    }
    let n = scan!();
    let p = scan!([usize; n]);
    let mut idx: Vec<_> = (0..n).collect();
    idx.sort_by_key(|&i| std::cmp::Reverse(p[i]));
    let mut bit = BinaryIndexedTree::new(n + 2, AdditiveOperation::new());
    bit.update(0, 1);
    bit.update(n + 1, 1);
    let mut ans = 0;
    for i in idx.into_iter() {
        bit.update(i + 1, 1);
        let cnt = bit.accumulate(i + 1);
        let l = bit.lower_bound(cnt - 1);
        let r = bit.lower_bound(cnt + 1);
        if l > 0 {
            ans += p[i] * (r - i - 1) * (l - bit.lower_bound(cnt - 2));
        }
        if r <= n {
            ans += p[i] * (i + 1 - l) * (bit.lower_bound(cnt + 2) - r);
        }
    }
    println!("{}", ans);
}