#if 1
#include <bits/stdc++.h>
const long long MOD = 998244353;

#include <atcoder/all>

using namespace atcoder;
typedef modint998244353 mint93;
typedef modint1000000007 mint17;
typedef modint mint;

using namespace std;
using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pll = pair<ll, ll>;
#define vec vector
template <class T>
using v = vector<T>;
template <class T>
using vv = v<v<T>>;
template <class T>
using vvv = v<vv<T>>;
using vl = v<ll>;
using vvl = vv<ll>;
using vvvl = vvv<ll>;
using vpl = v<pll>;
using UF = dsu;
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n - 1); }

#define FOR(i, a, b) for (ll i = (ll)(a); i < (ll)(b); i++)
#define rep(i, N) for (ll i = 0; i < (ll)(N); i++)
#define rep1(i, N) for (ll i = 1; i <= (ll)(N); i++)
#define rrep(i, N) for (ll i = N - 1; i >= 0; i--)
#define rrep1(i, N) for (ll i = N; i > 0; i--)
#define fore(i, a) for (auto &i : a)
#define fs first
#define sc second
#define eb emplace_back
#define pb push_back
#define ps push
#define elif else if
#define all(x) (x).begin(), (x).end()
#define UNIQUE(x) (x).erase(unique((x).begin(), (x).end()), (x).end());
#define YES(x) cout << ((x) ? "YES\n" : "NO\n");
#define Yes(x) cout << ((x) ? "Yes\n" : "No\n");
#define yes(x) cout << ((x) ? "yes\n" : "no\n");
#define sum(x) accumulate(all(x), 0ll)
template <class T, class U>
void chmin(T &t, const U &u) {
  if (t > u) t = u;
}
template <class T, class U>
void chmax(T &t, const U &u) {
  if (t < u) t = u;
}
template <class T>
T min(v<T> &lis) {
  return *min_element(all(lis));
}
template <class T>
T max(v<T> &lis) {
  return *max_element(all(lis));
}
const int inf = (1 << 29);
const ll infl = (1LL << 60);
const ll mod93 = 998244353ll;
const ll mod17 = 1000000007ll;
int popcnt(uint x) { return __builtin_popcount(x); }
int popcnt(ull x) { return __builtin_popcountll(x); }
int bsr(uint x) { return 32 - __builtin_clz(x); }
int bsr(ull x) { return 64 - __builtin_clzll(x); }
int bsf(uint x) { return __builtin_ctz(x); }
int bsf(ull x) { return __builtin_ctzll(x); }

template <class T>
istream &operator>>(istream &is, vector<T> &x) {
  for (auto &y : x) is >> y;
  return is;
}
template <class T>
ostream &operator<<(ostream &os, vector<T> &x) {
  for (unsigned int i = 0, size = x.size(); i < size; i++)
    os << x[i] << (i == size - 1 ? "" : " ");
  return os;
}
template <class T, class S>
istream &operator>>(istream &is, pair<T, S> &x) {
  return is >> x.first >> x.second;
}
template <class T, class S>
ostream &operator<<(ostream &os, pair<T, S> &x) {
  return os << x.first << " " << x.second;
}
template <class T>
T popleft(queue<T> &q){
    T a = q.front();
    q.pop();
    return a;
}

struct StopWatch {
  bool f = false;
  clock_t st;
  void start() {
    f = true;
    st = clock();
  }
  int msecs() {
    assert(f);
    return (clock() - st) * 1000 / CLOCKS_PER_SEC;
  }
};
ll rand_int(ll l, ll r) {  // [l, r]
  static random_device rd;
  static mt19937 gen(rd());
  return uniform_int_distribution<ll>(l, r)(gen);
}
template <long long MOD>
struct binomial {
  long long n;
  std::vector<atcoder::static_modint<MOD>> fact, ifact;
  binomial(long long n) : n(n), fact(n + 1), ifact(n + 1) {
    fact[0] = 1;
    for (int i = 1; i <= n; i++) fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i > 0; i--) ifact[i - 1] = ifact[i] * i;
  }
  atcoder::static_modint<MOD> operator()(int n, int k) {
    if (k < 0 || k > n) return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};
#endif

// #define _GLIBCXX_DEBUG


int main() {
  ll h, w;
  cin >> h >> w;
  vvl a(h - 2, vl(w));
  cin >> a;
  ll ans = infl;
  rep(i, h - 2){
    if (a[i][0] == -1) continue;
    queue<vl> q;
    q.push({i, 0, a[i][0]});
    while (!q.empty()){
      vl b = popleft(q);
      if (b[1] == w - 1){
        chmin(ans, b[2]);
        continue;
      }
      for (ll j = -1; j <= 1; j++) {
        if (h - 3 >= b[0] + j && b[0] + j >= 0 && a[b[0] + j][b[1] + 1] != -1){
          q.push({b[0] + j, b[1] + 1, b[2] + a[b[0] + j][b[1] + 1]});
        }
      }
    }
  }
  if (ans == infl) cout << -1 << endl;
  else cout << ans << endl;
}