#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;
}