H, W = map(int, input().split())
A = [list(map(int, input().split())) for _ in range(H - 2)]

d = [[float('inf')] * W for _ in range(H - 2)]
for h in range(H - 2):
    if A[h][0] == -1:
        continue
    d[h][0] = A[h][0]

for w in range(W - 1):
    for h in range(H - 2):
        if d[h][w] == float('inf'):
            continue
        for d_h in (-1, 0, 1):
            if h + d_h < 0 or h + d_h >= H - 2:
                continue
            if A[h + d_h][w + 1] == -1:
                continue
            d[h + d_h][w + 1] = min(d[h + d_h][w + 1], d[h][w] + A[h + d_h][w + 1])

ans = float('inf')
for h in range(H - 2):
    ans = min(ans, d[h][W - 1])

if ans == float('inf'):
    print(-1)
else:
    print(ans)