n,m=map(int,input().split())
a=[sum(list(map(int,input().split()))) for _ in range(n)]
ans=0
cnt=0
cnt2=0
for i in range(1,n):
    if cnt==0 and a[i-1]>a[i]:
        cnt=1
        ans+=a[i-1]
    elif cnt==1 and a[i-1]<a[i]:
        cnt=0
        cnt2=a[i-1]
        ans-=a[i-1]
if cnt==0:
    if cnt2<a[-1]:
        ans+=a[-1]
    else:
        ans+=cnt2
print(ans)