// SNIPPET slice
pub struct SliceGroupBy<'a, T: 'a, K: Eq, F: Fn(&T) -> K> {
    key_fn: F,
    rest: &'a [T],
}
impl<'a, T, K: Eq, F: Fn(&T) -> K> Iterator for SliceGroupBy<'a, T, K, F> {
    type Item = (K, &'a [T]);
    fn next(&mut self) -> Option<(K, &'a [T])> {
        if self.rest.is_empty() {
            return None;
        }
        let key = (self.key_fn)(&self.rest[0]);
        let mut end = 1;
        while end < self.rest.len() && (self.key_fn)(&self.rest[end]) == key {
            end += 1;
        }
        let (left, right) = self.rest.split_at(end);
        self.rest = right;
        Some((key, left))
    }
}
pub struct SplitByGap<'a, T: 'a, F: Fn(&T, &T) -> bool> {
    gap_fn: F,
    rest: &'a [T]
}
impl<'a, T, F: Fn(&T, &T) -> bool> Iterator for SplitByGap<'a, T, F> {
    type Item = &'a [T];
    fn next(&mut self) -> Option<&'a [T]> {
        if self.rest.is_empty() {
            return None;
        }
        let mut r = 1;
        while r < self.rest.len() && !(self.gap_fn)(&self.rest[r-1], &self.rest[r]) {
            r += 1;
        }
        let (result, rest) = self.rest.split_at(r);
        self.rest = rest;
        Some(result)
    }
}
pub struct Permutations<'a, T: 'a> {
    items: &'a [T],
    indices: Option<Vec<usize>>,
    is_first: bool,
}
impl<'a, T: 'a> Iterator for Permutations<'a, T> {
    type Item = Vec<&'a T>;
    fn next(&mut self) -> Option<Vec<&'a T>> {
        if !self.is_first {
            let indices_opt = self.indices.take();
            if let Some(indices) = indices_opt {
                self.indices = next_permutation(indices);
            }
        } else {
            self.is_first = false;
        }
        self.indices.as_ref().map(|indices| {
            indices.into_iter().map(|&i| &self.items[i]).collect()
        })
    }
}
fn next_permutation(mut indices: Vec<usize>) -> Option<Vec<usize>> {
    (0 .. indices.len().saturating_sub(1)).rev()
        .find(|&left| indices[left] < indices[left+1])
        .map(|left| {
            let right = (0..indices.len()).rev()
                .find(|&right| indices[left] < indices[right])
                .unwrap();
            indices.swap(left, right);
            indices[left+1..].reverse();
            indices
        })
}
fn count_inversions_sub<T: Clone + Ord>(seq: &[T]) -> (Vec<T>, usize) {
    if seq.len() <= 1 {
        (seq.to_vec(), 0)
    } else {
        let mid = seq.len() / 2;
        let (sub1, inv1) = count_inversions_sub(&seq[..mid]);
        let (sub2, inv2) = count_inversions_sub(&seq[mid..]);
        let mut sorted = Vec::new();
        let (mut i1, mut i2) = (0, 0);
        let mut inv = 0;
        loop {
            match (sub1.get(i1), sub2.get(i2)) {
                (Some(x1), Some(x2)) => {
                    if x1 <= x2 {
                        sorted.push(x1.clone());
                        i1 += 1;
                    } else {
                        inv += sub1.len() - i1;
                        sorted.push(x2.clone());
                        i2 += 1;
                    }
                },
                (Some(_), None) => {
                    sorted.extend(sub1[i1..].iter().cloned());
                    break;
                },
                (None, Some(_)) => {
                    sorted.extend(sub2[i2..].iter().cloned());
                    break;
                },
                (None, None) => break,
            }
        }
        (sorted, inv + inv1 + inv2)
    }
}
pub trait SliceExt<T> {
    fn group_by<K: Eq, F: Fn(&T) -> K>(&self, key_fn: F) -> SliceGroupBy<T, K, F>;
    fn split_by_gap<F: Fn(&T, &T) -> bool>(&self, gap_fn: F) -> SplitByGap<T, F>;
    fn permutations(&self) -> Permutations<T>;
    fn count_inversions(&self) -> usize where T: Clone + Ord;
}
impl<T> SliceExt<T> for [T] {
    fn group_by<K: Eq, F: Fn(&T) -> K>(&self, key_fn: F) -> SliceGroupBy<T, K, F> {
        SliceGroupBy { key_fn: key_fn, rest: self }
    }
    fn split_by_gap<F: Fn(&T, &T) -> bool>(&self, gap_fn: F) -> SplitByGap<T, F> {
        SplitByGap { gap_fn: gap_fn, rest: self }
    }
    fn permutations(&self) -> Permutations<T> {
        let indices = if self.is_empty() {
            None
        } else {
            Some((0..self.len()).collect())
        };
        Permutations {
            items: self,
            indices: indices,
            is_first: true,
        }
    }
    fn count_inversions(&self) -> usize where T: Clone + Ord {
        count_inversions_sub(self).1
    }
}
pub trait SliceOfVecsExt<T> {
    fn transpose_clone(&self) -> Option<Vec<Vec<T>>> where T: Clone;
}
impl<T> SliceOfVecsExt<T> for [Vec<T>] {
    fn transpose_clone(&self) -> Option<Vec<Vec<T>>> where T: Clone {
        if self.iter().any(|row| row.is_empty()) {
            return None;
        }
        if self.windows(2).any(|rows| rows[0].len() < rows[1].len()) {
            return None;
        }
        let mut result = Vec::new();
        let n = self.get(0).map_or(0, |first| first.len());
        for i in 0..n {
            let mut result_row = Vec::new();
            for j in 0..self.len() {
                if self[j].len() <= i {
                    break;
                }
                result_row.push(self[j][i].clone());
            }
            result.push(result_row);
        }
        Some(result)
    }
}
// SNIPPET read
pub trait Readable {
    type Output;
    fn words_count() -> usize;
    fn read_words(words: &[&str]) -> Result<Self::Output, String>;
}
#[macro_export]
macro_rules! readable {
    ( $t:ty, $words_count:expr, |$words:ident| $read_words:expr ) => {
        impl Readable for $t {
            type Output = $t;
            fn words_count() -> usize { $words_count }
            fn read_words($words: &[&str]) -> Result<$t, String> {
                Ok($read_words)
            }
        }
    };
}
readable!((), 1, |_ss| ());
readable!(String, 1, |ss| ss[0].to_string());
impl Readable for char {
    type Output = char;
    fn words_count() -> usize { 1 }
    fn read_words(words: &[&str]) -> Result<char, String> {
        let chars: Vec<char> = words[0].chars().collect();
        if chars.len() == 1 {
            Ok(chars[0])
        } else {
            Err(format!("cannot parse `{}` as a char", words[0]))
        }
    }
}
pub struct Chars();
impl Readable for Chars {
    type Output = Vec<char>;
    fn words_count() -> usize { 1 }
    fn read_words(words: &[&str]) -> Result<Vec<char>, String> {
        Ok(words[0].chars().collect())
    }
}
macro_rules! impl_readable_for_ints {
    ( $( $t:ty )* ) => { $(
        impl Readable for $t {
            type Output = Self;
            fn words_count() -> usize { 1 }
            fn read_words(words: &[&str]) -> Result<$t, String> {
                use std::str::FromStr;
                <$t>::from_str(words[0]).map_err(|_| {
                    format!("cannot parse `{}` as {}", words[0], stringify!($t))
                })
            }
        }
    )* };
}
impl_readable_for_ints!(i8 u8 i16 u16 i32 u32 i64 u64 isize usize f32 f64);
macro_rules! define_one_origin_int_types {
    ( $new_t:ident $int_t:ty ) => {
        #[allow(non_camel_case_types)]
        pub struct $new_t;
        impl Readable for $new_t {
            type Output = $int_t;
            fn words_count() -> usize { 1 }
            fn read_words(words: &[&str]) -> Result<Self::Output, String> {
                <$int_t>::read_words(words).map(|n| n-1)
            }
        }
    };
    ( $new_t:ident $int_t:ty; $( $inner_new_t:ident $inner_int_t:ty );* ) => {
        define_one_origin_int_types!($new_t $int_t);
        define_one_origin_int_types!($($inner_new_t $inner_int_t);*);
    };
}
define_one_origin_int_types!(u8_ u8; u16_ u16; u32_ u32; u64_ u64; usize_ usize);
macro_rules! impl_readable_for_tuples {
    ( $t:ident $var:ident ) => ();
    ( $t:ident $var:ident; $( $inner_t:ident $inner_var:ident );* ) => {
        impl_readable_for_tuples!($($inner_t $inner_var);*);
        impl<$t: Readable, $($inner_t: Readable),*> Readable
            for ($t, $($inner_t),*)
        {
            type Output = ( <$t>::Output, $(<$inner_t>::Output),* );
            fn words_count() -> usize {
                let mut n = <$t>::words_count();
                $(
                    n += <$inner_t>::words_count();
                )*
                n
            }
            #[allow(unused_assignments)]
            fn read_words(words: &[&str]) ->
                Result<Self::Output, String>
            {
                let mut start = 0;
                let $var = <$t>::read_words(
                    &words[start .. start+<$t>::words_count()]
                )?;
                start += <$t>::words_count();
                $(
                    let $inner_var =
                        <$inner_t>::read_words(
                            &words[start .. start+<$inner_t>::words_count()]
                        )?;
                    start += <$inner_t>::words_count();
                )*
                Ok(( $var, $($inner_var),* ))
            }
        }
    };
}
impl_readable_for_tuples!(T8 x8; T7 x7; T6 x6; T5 x5; T4 x4; T3 x3; T2 x2; T1 x1);
pub trait ReadableFromLine {
    type Output;
    fn read_line(line: &str) -> Result<Self::Output, String>;
}
fn split_into_words(line: &str) -> Vec<&str> {
    #[allow(deprecated)]
    line.trim_right_matches('\n').split_whitespace().collect()
}
impl<T: Readable> ReadableFromLine for T {
    type Output = T::Output;
    fn read_line(line: &str) -> Result<T::Output, String> {
        let words = split_into_words(line);
        if words.len() != T::words_count() {
            return Err(format!("line `{}` has {} words, expected {}",
                               line, words.len(), T::words_count()));
        }
        T::read_words(&words)
    }
}
macro_rules! impl_readable_from_line_for_tuples_with_from_iterator {
    ( $u:ident : $( + $bound:path )* => $seq_in:ty, $seq_out:ty; $t:ident $var:ident ) => {
        impl<$u: Readable> ReadableFromLine for $seq_in
        where
            <$u as Readable>::Output: Sized $(+ $bound)*
        {
            type Output = $seq_out;
            fn read_line(line: &str) -> Result<$seq_out, String> {
                let n = $u::words_count();
                let words = split_into_words(line);
                if words.len() % n != 0 {
                    return Err(format!("line `{}` has {} words, expected multiple of {}",
                                       line, words.len(), n));
                }
                let mut result = Vec::new();
                for chunk in words.chunks(n) {
                    match $u::read_words(chunk) {
                        Ok(v) => result.push(v),
                        Err(msg) => {
                            let flagment_msg = if n == 1 {
                                format!("word {}", result.len())
                            } else {
                                let l = result.len();
                                format!("words {}-{}", n*l + 1, (n+1) * l)
                            };
                            return Err(format!(
                                "{} of line `{}`: {}", flagment_msg, line, msg
                            ));
                        }
                    }
                }
                Ok(result.into_iter().collect())
            }
        }
        impl<T: Readable, $u: Readable> ReadableFromLine for (T, $seq_in)
        where
            <$u as Readable>::Output: Sized $(+ $bound)*
        {
            type Output = (T::Output, $seq_out);
            fn read_line(line: &str) -> Result<Self::Output, String> {
                let n = T::words_count();
                #[allow(deprecated)]
                let trimmed = line.trim_right_matches('\n');
                let words_and_rest: Vec<&str> = trimmed.splitn(n + 1, ' ').collect();
                if words_and_rest.len() < n {
                    return Err(format!("line `{}` has {} words, expected at least {}",
                                       line, words_and_rest.len(), n));
                }
                let words = &words_and_rest[..n];
                let empty_str = "";
                let rest = words_and_rest.get(n).unwrap_or(&empty_str);
                Ok((T::read_words(words)?, <$seq_in>::read_line(rest)?))
            }
        }
    };
    ( $u:ident : $( + $bound:path )* => $seq_in:ty, $seq_out:ty;
      $t:ident $var:ident, $( $inner_t:ident $inner_var:ident ),+ ) => {
        impl_readable_from_line_for_tuples_with_from_iterator!(
            $u: $(+ $bound)* => $seq_in, $seq_out; $($inner_t $inner_var),+
        );
        impl<$t: Readable, $($inner_t: Readable),+ , $u: Readable> ReadableFromLine
            for ($t, $($inner_t),+ , $seq_in)
        where
            <$u as Readable>::Output: Sized $(+ $bound)*
        {
            type Output = ($t::Output, $($inner_t::Output),+ , $seq_out);
            fn read_line(line: &str) -> Result<Self::Output, String> {
                let mut n = $t::words_count();
                $(
                    n += $inner_t::words_count();
                )+
                #[allow(deprecated)]
                let trimmed = line.trim_right_matches('\n');
                let words_and_rest: Vec<&str> = trimmed.splitn(n + 1, ' ')
                    .collect();
                if words_and_rest.len() < n {
                    return Err(
                        format!("line `{}` has {} words, expected at least {}",
                                line, words_and_rest.len(), n)
                    );
                }
                let words = &words_and_rest[..n];
                let empty_str = "";
                let rest = words_and_rest.get(n).unwrap_or(&empty_str);
                let ($var, $($inner_var),*) =
                    <($t, $($inner_t),+)>::read_words(words)?;
                Ok(($var, $($inner_var),* , <$seq_in>::read_line(rest)?))
            }
        }
    };
}
#[macro_export]
macro_rules! readable_collection {
    ($u:ident => $collection_in:ty, $collection_out:ty) => {
        impl_readable_from_line_for_tuples_with_from_iterator!(
            $u: => $collection_in, $collection_out;
            T8 x8, T7 x7, T6 x6, T5 x5, T4 x4, T3 x3, T2 x2, T1 x1
        );
    };
    ($u:ident : $( $bound:path ),* => $collection_in:ty, $collection_out:ty) => {
        impl_readable_from_line_for_tuples_with_from_iterator!(
            $u: $(+ $bound)* => $collection_in, $collection_out;
            T8 x8, T7 x7, T6 x6, T5 x5, T4 x4, T3 x3, T2 x2, T1 x1
        );
    }
}
readable_collection!(U => Vec<U>, Vec<U::Output>);
readable_collection!(
    U => std::collections::VecDeque<U>, std::collections::VecDeque<U::Output>
);
readable_collection!(
    U: Eq, std::hash::Hash => std::collections::HashSet<U>, std::collections::HashSet<U::Output>
);
readable_collection!(
    U: Ord => std::collections::BTreeSet<U>, std::collections::BTreeSet<U::Output>
);
readable_collection!(
    U: Ord => std::collections::BinaryHeap<U>, std::collections::BinaryHeap<U::Output>
);
pub fn read<T: ReadableFromLine>() -> T::Output {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line).unwrap();
    T::read_line(&line).unwrap()
}
#[macro_export]
macro_rules! read {
    () => {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
    };
    ( $pat:pat = $t:ty ) => {
        let $pat = read::<$t>();
    };
    ( $( $pat:pat = $t:ty ),+ ) => {
        read!(($($pat),*) = ($($t),*));
    };
}
#[macro_export]
macro_rules! readls {
    ( $( $pat:pat = $t:ty ),+ ) => {
        $(
            read!($pat = $t);
        )*
    };
}
pub fn readx<T: ReadableFromLine>() -> Vec<T::Output> {
    use std::io::{self, BufRead};
    let stdin = io::stdin();
    let result = stdin.lock().lines().map(|line_result| {
        let line = line_result.expect("read from stdin failed");
        T::read_line(&line).unwrap()
    }).collect();
    result
}
#[macro_export]
macro_rules! readx_loop {
    ( |$pat:pat = $t:ty| $body:expr ) => {
        {
            use std::io::BufRead;
            let stdin = std::io::stdin();
            for line in stdin.lock().lines() {
                let line = line.expect("read from stdin failed");
                let $pat = <$t>::read_line(&line).unwrap();
                $body
            }
        }
    };
    ( |$($pat:pat = $t:ty),*| $body:expr ) => {
        readx_loop!(|($($pat),*) = ($($t),*)| $body);
    };
}
pub fn readn<T: ReadableFromLine>(n: usize) -> Vec<T::Output> {
    use std::io::{self, BufRead};
    let stdin = io::stdin();
    let result: Vec<T::Output> = stdin.lock().lines().take(n).map(|line_result| {
        let line = line_result.expect("read from stdin failed");
        T::read_line(&line).unwrap()
    }).collect();
    if result.len() < n {
        panic!("expected reading {} lines, but only {} lines are read",
               n, result.len());
    }
    result
}
#[macro_export]
macro_rules! readn_loop {
    ( $n:expr, |$pat:pat = $t:ty| $body:expr ) => {
        {
            use std::io::BufRead;
            let stdin = std::io::stdin();
            let mut lock = stdin.lock();
            for _ in 0..$n {
                let mut line = String::new();
                lock.read_line(&mut line).expect("read from stdin failed");
                let $pat = <$t>::read_line(&line).unwrap();
                $body
            }
        }
    };
    ( $n:expr, |$($pat:pat = $t:ty),*| $body:expr ) => {
        readn_loop!($n, |($($pat),*) = ($($t),*)| $body);
    };
}
pub trait Words {
    fn read<T: Readable>(&self) -> T::Output;
}
impl<'a> Words for [&'a str] {
    fn read<T: Readable>(&self) -> T::Output {
        T::read_words(self).unwrap()
    }
}
impl<'a> Words for &'a str {
    fn read<T: Readable>(&self) -> T::Output {
        T::read_words(&[self]).unwrap()
    }
}
fn main() {
    read!(n = usize);
    let aa = readx::<u64>();
    if aa[0] != 0 || aa.windows(2).any(|w| w[1] > w[0] + 1) {
        println!("-1");
        return;
    }

    let ans: u64 = aa.split_by_gap(|&a, &b| a > b).map(|seq| {
        let mut iter = seq.iter().rev();
        let first = *iter.next().unwrap();
        iter.fold((first, first), |(count, last), &a| {
            if last == a {
                (count + a, a)
            } else {
                (count, a)
            }
        }).0
    }).sum();
    println!("{}", ans);
}


Measured memory_bytes: 6396