H,W = map(int,input().split())
A = []
A.append([-1]*W)
for _ in range(H-2):
    A.append(list(map(int,input().split())))
A.append([-1]*W)

INF = float('inf')
dp = [[INF]*W for _ in range(H)]
for i in range(H):
    if A[i][0] == -1: continue
    dp[i][0] = A[i][0]

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

ans = INF
for i in range(H):
    ans = min(ans, dp[i][-1])
print(-1 if ans==INF else ans)