using namespace std;
#include<bits/stdc++.h>
void _main(); int main() { cin.tie(0); ios::sync_with_stdio(false); cout<<fixed<<setprecision(15);_main(); return 0;}
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
typedef long long ll;
typedef long double ld;
template<class ll> inline bool chmax(ll& a, ll b) { if (a < b) { a = b; return 1; } return 0; }
template<class ll> inline bool chmin(ll& a, ll b) { if (a > b) { a = b; return 1; } return 0; }
#define rep(i, star,fini) for (int i = star; i < fini; i++)
#define ALL(x) std::begin(x), std::end(x)
#define INF ((1LL<<62)-(1LL<<31))
#define bit(x,i)(((x)>>(i))&1)
long double distance(long double xi,long double yi,long double xj,long double yj){return sqrt(1.0*((xi-xj)*(xi-xj))+1.0*((yi-yj)*(yi-yj)));}
ll m[1000][1000],dp[1000][1000];
void _main(){
    ll h,w;cin >> h >> w;
    rep(i,0,h)rep(j,0,w)m[i][j]=-1,dp[i][j]=INF;
    rep(i,1,h-1)rep(j,0,w)cin >> m[i][j];
    
    rep(j,0,h)dp[j][0]=0;
    rep(i,1,h-1)dp[i][0]=m[i][0];
    rep(i,1,w){
        rep(j,1,h-1){
            if(m[j][i]!=-1){
                if(m[j-1][i-1]!=-1)chmin(dp[j][i],dp[j-1][i-1]+m[j][i]);
                if(m[j][i-1]!=-1)chmin(dp[j][i],dp[j][i-1]+m[j][i]);
                if(m[j+1][i-1]!=-1)chmin(dp[j][i],dp[j+1][i-1]+m[j][i]);
            }
        }
    }
    ll ans = INF;
    rep(i,0,h){
        chmin(ans,dp[i][w-1]);
    }
    cout<<(ans!=INF?ans:-1)<<endl;
}