from heapq import heappop, heappush


h, w = map(int, input().split())
a = [list(map(int, input().split())) for _ in range(h - 2)]

# 最小カットするのは面倒なので、左から右にpathを通して切る
distance = [[-1] * w for _ in range(h - 2)]
heap_queue = []

for i in range(h - 2):
    if a[i][0] >= 0:
        heappush(heap_queue, ([i][0], i, 0))

while len(heap_queue):
    d, i0, j0 = heappop(heap_queue)
    if distance[i0][j0] == -1:
        distance[i0][j0] = d
    else:
        continue
    
    # 8方向にpath
    for i1 in range(i0 - 1, i0 + 2):
        for j1 in range(j0 - 1, j0 + 2):
            if i1 == i0 and j1 == j0:
                continue
            if 0 <= i1 < h - 2 and 0 <= j1 < w:
                if a[i1][j1] >= 0 and distance[i1][j1] == -1:
                    heappush(heap_queue, (distance[i0][j0] + a[i1][j1], i1, j1))

res_list = [distance[i][-1] for i in range(h - 2)]
if max(res_list) == -1:
    print(-1)
else:
    print(max([r for r in res_list if r >= 0]))