const

maxn=1001;

var

a,f:array [-1..maxn,-1..maxn] of longint;

i,j,n,m,k,l,max:longint;

begin

read(n,m);

for i:=1 to n do

for j:=1 to m do

read(a[i,j]);

f[1,1]:=a[1,1];

for i:=2 to n do

for j:=2 to m do

if a[i,j]=-1 then continue

else

begin

if (i-2>0) and (j-1>0) then

if a[i-2,j-1]<>-1 then

if f[i,j]<f[i-2,j-1]+a[i,j] then

f[i,j]:=f[i-2,j-1]+a[i,j];

if (i-1>0) and (j-2>0) then

if a[i-1,j-2]<>-1 then

if f[i,j]<f[i-1,j-2]+a[i,j] then

f[i,j]:=f[i-1,j-2]+a[i,j]

end;

max:=0;

for i:=1 to m do

if f[n,i]>max then max:=f[n,i];

for i:=1 to n do

if f[i,m]>max then max:=f[i,m];

if max>0 then writeln(max) else writeln('cannot arrive')

end.