#include <iostream>
#include <algorithm>
#include <utility>
#include <numeric>
#include <vector>
#include <array>

template <class T, class U>
inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) { lhs = rhs; return true; }
  return false;
}

template <class T, class U>
inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) { lhs = rhs; return true; }
  return false;
}

struct range {
  using itr = int64_t;
  struct iterator {
    itr i;
    constexpr iterator(itr i_) noexcept : i(i_) { }
    constexpr void operator ++ () noexcept { ++i; }
    constexpr itr operator * () const noexcept { return i; }
    constexpr bool operator != (iterator x) const noexcept { return i != x.i; }
  };
  const iterator l, r;
  constexpr range(itr l_, itr r_) noexcept : l(l_), r(std::max(l_, r_)) { }
  constexpr iterator begin() const noexcept { return l; }
  constexpr iterator end() const noexcept { return r; }
};

struct revrange {
  using itr = int64_t;
  struct iterator {
    itr i;
    constexpr iterator(itr i_) noexcept : i(i_) { }
    constexpr void operator ++ () noexcept { --i; }
    constexpr itr operator * () const noexcept { return i; }
    constexpr bool operator != (iterator x) const noexcept { return i != x.i; }
  };
  const iterator l, r;
  constexpr revrange(itr l_, itr r_) noexcept : l(l_ - 1), r(std::max(l_, r_) - 1) { }
  constexpr iterator begin() const noexcept { return r; }
  constexpr iterator end() const noexcept { return l; }
};

using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;

constexpr i32 inf32 = (i32(1) << 30) - 1;
constexpr i64 inf64 = (i64(1) << 62) - 1;

int main() {
  size_t N;
  std::cin >> N;
  std::vector<i32> A(N);
  for (auto &x: A) {
    std::cin >> x;
  }
  std::vector<i32> prefmax(N), prefmin(N), suffmax(N), suffmin(N);
  for (auto i: range(0, N)) {
    prefmax[i] = A[i];
    prefmin[i] = A[i];
    if (i > 0) {
      chmax(prefmax[i], prefmax[i - 1]);
      chmin(prefmin[i], prefmin[i - 1]);
    }
  }
  for (auto i: revrange(0, N)) {
    suffmax[i] = A[i];
    suffmin[i] = A[i];
    if (i + 1 < N) {
      chmax(suffmax[i], suffmax[i + 1]);
      chmin(suffmin[i], suffmin[i + 1]);
    }
  }
  i32 ans = inf32;
  for (auto i: range(1, N - 1)) {
    if (prefmax[i - 1] > A[i] && suffmax[i + 1] > A[i]) {
      chmin(ans, prefmax[i - 1] + A[i] + suffmax[i + 1]);
    }
    if (prefmin[i - 1] < A[i] && suffmin[i + 1] < A[i]) {
      chmin(ans, prefmin[i - 1] + A[i] + suffmin[i + 1]);
    }
  }
  std::cout << (ans == inf32 ? -1 : ans) << '\n';
  return 0;
}