#include <bits/stdc++.h>
using namespace std;
using ll = long long;
constexpr ll INF64 = 1LL<<60;
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

int main() {
    int h, w;
    cin >> h >> w;
    h -= 2;
    vector a(h, vector(w, 0ll));
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            cin >> a[i][j];
            if (a[i][j] == -1) {
                a[i][j] = INF64;
            }
        }
    }
    vector dp(h, vector(w, INF64));
    for (int i = 0; i < h; i++) {
        dp[i][0] = a[i][0];
    }
    for (int j = 0; j < w - 1; j++) {
        for (int i = 0; i < h; i++) {
            for (const auto diff : { -1, 0, 1 }) {
                auto nxt = i + diff;
                if (0 <= nxt && nxt < h) {
                    chmin(dp[nxt][j + 1], a[nxt][j + 1] + dp[i][j]);
                }
            }
        }
    }
    ll ans = INF64;
    for (int i = 0; i < h; i++) {
        chmin(ans, dp[i][w - 1]);
    }
    cout << (ans == INF64 ? -1 : ans) << endl;
    return 0;
}