#include <bits/stdc++.h>   using namespace std;   int n, m;   long long sum[1505][1505];   long long ans[1505];   long long t[1505];   int main() {     int i, j, x;     long long val, now, res;     scanf( %d%d , &n, &m);     for (i = 1; i <= n; i++)       for (j = 1; j <= m; j++)         scanf( %I64d , &val), sum[i][j] = sum[i][j - 1] + val;     for (i = 1; i <= m; i++) ans[i] = sum[1][i];     for (i = 2; i <= n; i++) {       if (i % 2 == 0) {         now = -(long long)1e15;         for (x = m; x > 0; x--) {           t[x] = now + sum[i][x];           now = max(now, ans[x]);         }       } else {         now = -(long long)1e15;         for (x = 1; x <= m; x++) {           t[x] = now + sum[i][x];           now = max(now, ans[x]);         }       }       for (x = 1; x <= m; x++) ans[x] = t[x];     }     res = -(long long)1e15;     for (x = 1; x <= m; x++) res = max(res, ans[x]);     printf( %I64d , res);   }   