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