from bisect import bisect

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

DP=[[1<<63]*W for i in range(H-2)]

for i in range(H-2):
    if A[i][0]!=-1:
        DP[i][0]=A[i][0]

for i in range(1,W):
    for j in range(H-2):
        if A[j][i]==-1:
            continue

        if j-1>=0:
            DP[j][i]=min(DP[j][i],DP[j-1][i-1]+A[j][i])
            
        DP[j][i]=min(DP[j][i],DP[j][i-1]+A[j][i])

        if j+1<H-2:
            DP[j][i]=min(DP[j][i],DP[j+1][i-1]+A[j][i])

ANS=1<<63

for i in range(H-2):
    ANS=min(ANS,DP[i][-1])

if ANS==1<<63:
    print(-1)
else:
    print(ANS)