#include <iostream>
#include <vector>
using namespace std;
const int maxn = 1005;

int dp[maxn][maxn][2];
int main()
{
    ios_base::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;

    vector<vector<int>> mat(n, vector<int>(m));

    for(int i = 0; i < n; i++) {
        for(int j = 0; j < m; j++) {
            cin >> mat[i][j];
        }
    }

    for(int i = 0; i < n; i++) {
        int max1 = 0, max2 = 0, idx = 0;
        for(int j = 0; j < m; j++) {
            dp[i][j][0] = mat[i][j];
            dp[i][j][1] = dp[i][j][0];
            dp[1][0][0] = 0;

            if(i > 0) {
                dp[i][j][0] += dp[i - 1][j][1];
            }

            if(dp[i][j][0] > max1) {
                max2 = max1;
                max1 = dp[i][j][0];
                idx = j;
            }
            else if(dp[i][j][0] > max2) {
                max2 = dp[i][j][0];
            }
        }

        for(int j = 0; j < m; j++) {
            if(i == 0) {
                if(j != 0) {
                    dp[0][j][1] += dp[0][0][0];
                }
            }
            else {
                if(j == idx) {
                    dp[i][j][1] += max2;
                }
                else {
                    dp[i][j][1] += max1;
                }
            }
        }
    }

    cout << max(dp[n - 1][m - 1][0], dp[n - 1][m - 1][1]) << endl;


    return 0;
}