from sys import setrecursionlimit, stdin
from collections import defaultdict, deque, Counter
from itertools import permutations, combinations, product
from functools import lru_cache
from bisect import bisect_left, bisect_right
from heapq import heappush, heappop
from copy import copy, deepcopy
from decimal import Decimal
from random import random, randrange

# from pypyjit import set_param
# set_param('max_unroll_recursion=-1')

setrecursionlimit(1 << 20)
readline = stdin.readline
INF = 10 ** 18
MOD = 998244353
# MOD = 1000000007
ALP = 26
''' Input '''
def I(): return int(readline())
def ST(): return readline()[:-1]
def LI(): return list(map(int, readline().split()))
def LII(): return list(map(lambda x: int(x) - 1, readline().split()))
def LF(x, func): return [func() for _ in [0] * x]
def SPI(): return map(int, readline().split())
def SPII(): return map(lambda x: int(x) - 1, readline().split())
def FIE(x): return [readline()[:-1] for _ in [0] * x]

''' Array '''
def cmin(dp, i, x):
    if x < dp[i]: dp[i] = x
def cmax(dp, i, x):
    if x > dp[i]: dp[i] = x

''' Alphabet '''
def alp_a_to_i(s): return ord(s) - ord('a')
def alp_A_to_i(s): return ord(s) - ord('A')
def alp_i_to_a(i): return chr(ord('a') + i)
def alp_i_to_A(i): return chr(ord('A') + i)

''' Other'''
def nynx(y, x, H, W): return [(y + dy, x + dx) for dy, dx in [(-1, 0), (1, 0), (0, -1), (0, 1)] if 0 <= y + dy < H and 0 <= x + dx < W]
def gen(x, *args):
    if len(args) == 1: return [x] * args[0]
    if len(args) == 2: return [[x] * args[1] for _ in [0] * args[0]]
    if len(args) == 3: return [[[x] * args[2] for _ in [0] * args[1]] for _ in [0] * args[0]]
    if len(args) == 4: return [[[[x] * args[3] for _ in [0] * args[2]] for _ in [0] * args[1]] for _ in [0] * args[0]]

''' Output '''
def pprint(E): 
    print()
    for e in E: print(e)
def Yes(): print("Yes")
def No(): print("No")
def YES(): print("YES")
def NO(): print("NO")
def yn(x): print("Yes" if x else "No")
def YN(x): print("YES" if x else "NO")
###############################################################################################

H, W = SPI()
A = LF(H - 2, LI)
A = [[-1] * W] + A + [[-1] * W]

dp = gen(INF, H, W)
for i in range(H):
    if A[i][0] != -1:
        dp[i][0] = A[i][0]

for j in range(1, W):
    for i in range(1, H - 1):
        if A[i][j] == -1: continue
        dp[i][j] = min(dp[i + 1][j - 1], dp[i][j - 1], dp[i - 1][j - 1]) + A[i][j]
        dp[i][j] = min(dp[i][j], INF)

# pprint(A)
# pprint(dp)
ans = INF
for i in range(H):
    ans = min(ans, dp[i][W - 1])

if ans == INF: ans = -1
print(ans)