Pioneer

#include<bits/stdc++.h>
using namespace std;

int n,m;

int arr[2010][2010];
int dp[2010][2010];

int cal(int I,int J){

//  int ans = arr[I][J];
    if(dp[I][J] != -1)return dp[I][J];
    int a,b,c,d;
    a=b=c=d=0;
    if(I%2 == 0){
        if(I+1 < n && arr[I+1][J] != -1)a = cal(I+1,J);//+arr[I][J];
        if(J+1 < m && arr[I][J+1] != -1)b = cal(I,J+1);//+arr[I][J];
    }
    else{
        if(I+1 < n && arr[I+1][J] != -1)c = cal(I+1,J);//+arr[I][J];
        if(J-1 >= 0 && arr[I][J-1] != -1)d = cal(I,J-1);//+arr[I][J];
    }
    return dp[I][J] = arr[I][J] + max(max(a,b),max(c,d));
}

int main()
{
    //freopen("input.txt","r",stdin);
    scanf("%d %d",&n,&m);
    for(int i = 0 ; i < n ; i ++){
        for(int j = 0 ; j < m ; j ++){
            scanf("%d",&arr[i][j]);
            dp[i][j] = -1;
        }
    }
    int ans = 0;
    for(int i = 0 ; i < n ; i ++){
        for(int j = 0 ; j < m ; j ++){
            ans = max(ans,cal(i,j));
        }
    }
    printf("%d",ans);
    return 0;
}