N,M=map(int,input().split())
A=[sum(list(map(int,input().split()))) for i in range(N)]
DP=[[0,0] for i in range(N+1)]
for i in range(N):
  DP[i+1][1]=max(DP[i][1],DP[i][0]+A[i])
  DP[i+1][0]=max(DP[i][0],DP[i][1]-A[i])
print(max(DP[N]))