#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:268435456")
using namespace std;
template <typename T>
inline T abs(T a) {
  return ((a < 0) ? -a : a);
}
template <typename T>
inline T sqr(T a) {
  return a * a;
}
template <class T>
T gcd(T a, T b) {
  return a ? gcd(b % a, a) : b;
}
template <class T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
template <class T>
T sign(T a) {
  return a > 0 ? 1 : (a < 0 ? -1 : 0);
}
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, 1, 0, -1};
const int dxK[] = {-1, -1, 0, 1, 1, 1, 0, -1};
const int dyK[] = {0, 1, 1, 1, 0, -1, -1, -1};
const int dxKn[] = {-2, -1, 1, 2, 2, 1, -1, -2};
const int dyKn[] = {1, 2, 2, 1, -1, -2, -2, -1};
const int N = int(2e3) + 9;
const int M = N * N / 2;
const int LOGN = 22;
const int SQN = 350;
const int MOD = int(1e9) + 7;
const int INF = 1e9 + 100;
const long long INF64 = 2e18;
const long double PI = 3.1415926535897932384626433832795;
const long double EPS = 1e-9;
int n, m;
int a[N][N];
long long dp[N][N];
void solve() {
  for (int i = 1; i < (int)(N); ++i)
    for (int j = 0; j < (int)(N); ++j) dp[i][j] = -INF64;
  scanf("%d %d", &n, &m);
  for (int i = 1; i < (int)(n + 1); ++i)
    for (int j = 1; j < (int)(m + 1); ++j) scanf("%d", &a[i][j]);
  for (int i = 1; i < (int)(n + 1); ++i) {
    if (i % 2 == 1) {
      long long sum = a[i][1];
      for (int j = 2; j <= m; ++j) {
        sum += a[i][j];
        dp[i][j] = sum + dp[i - 1][j - 1];
      }
      for (int j = m - 1; j >= 1; --j) dp[i][j] = max(dp[i][j], dp[i][j + 1]);
    } else {
      long long sum = 0;
      for (int j = 1; j < m; ++j) sum += a[i][j];
      for (int j = m - 1; j >= 1; --j) {
        dp[i][j] = sum + dp[i - 1][j + 1];
        sum -= a[i][j];
      }
      for (int j = 2; j <= m; ++j) dp[i][j] = max(dp[i][j], dp[i][j - 1]);
    }
  }
  long long res = dp[n][1];
  for (int j = 1; j <= m; ++j) res = max(dp[n][j], res);
  cout << res;
}
int main() {
  srand(time(NULL));
  cout << setprecision(10) << fixed;
  cerr << setprecision(10) << fixed;
  solve();
  return 0;
}
