#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
  int H,W;
  cin>>H>>W;
  H -= 2;
  vector<vector<int>> A(H,vector<int>(W));
  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] = 1e18;
  } 
  vector<vector<int>> dp(H,vector<int>(W,1e18));
  for(int i=0;i<H;i++) dp[i][0] = A[i][0];
  for(int i=0;i<W-1;i++){
  	for(int j=0;j<H;j++){
  		dp[j][i+1] = min(dp[j][i+1],dp[j][i]+A[j][i+1]);
  		if(j != H-1) dp[j+1][i+1] = min(dp[j+1][i+1],dp[j][i]+A[j+1][i+1]);
  		if(j) dp[j-1][i+1] = min(dp[j-1][i+1],dp[j][i]+A[j-1][i+1]);
  	}
  }
  int ans = 1e18;
  for(int i=0;i<H;i++) ans = min(ans,dp[i][W-1]);
  cout<<(ans == 1e18 ? -1 : ans)<<endl;
}