/* -*- coding: utf-8 -*-
 *
 * 2328.cc:  No.2328 Build Walls - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_H = 800;
const int MAX_W = 800;
const int INF = 1 << 30;

/* typedef */

/* global variables */

int as[MAX_H][MAX_W], dp[MAX_H][MAX_W];

/* subroutines */

/* main */

int main() {
  int h, w;
  scanf("%d%d", &h, &w);
  h -= 2;

  for (int i = 0; i < h; i++)
    for (int j = 0; j < w; j++) scanf("%d", as[i] + j);
  
  for (int i = 0; i < h; i++)
    dp[i][0] = (as[i][0] >= 0) ? as[i][0] : INF;
  
  for (int j = 1; j < w; j++)
    for (int i = 0; i < h; i++) {
      if (as[i][j] >= 0) {
	int d0 = (i > 0) ? dp[i - 1][j - 1] : INF;
	int d1 = dp[i][j - 1];
	int d2 = (i + 1 < h) ? dp[i + 1][j - 1] : INF;
	int d = min(min(d0, d1), d2);
	dp[i][j] = (d < INF) ? d + as[i][j] : INF;
      }
      else
	dp[i][j] = INF;
    }

  int mind = INF;
  for (int i = 0; i < h; i++)
    mind = min(mind, dp[i][w - 1]);

  printf("%d\n", (mind < INF) ? mind : -1);
  return 0;
}