#include <fstream.h>

int main()
{  long int n,m,i,j,a[100][100],b[4],max=0,s=0,c=0;
	ifstream fin("flip.in");
   fin>>n>>m;
   fin.close();
   for(i=1;i<=n;i++)
   	for(j=1;j<=m;j++)
      {
      	fin>>a[i][j];

         if (a[i][j]>=0) s+=a[i][j];
      	else s=s+(-1)*a[i][j];
      }
	b[1]=a[1][m];
   b[2]=a[n][1];
   b[3]=a[1][1];
	b[4]=a[n][m];
   for(i=1;i<=4;i++)
   	if (b[i]<0)
      {
      	if(!c)
         	{max=b[i]; c=1;}
         else
         {	if (max<b[i])
            	max=b[i];
         }
      }
   if(c) s+=max;
   ofstream fout("flip.out");
   fout<<s;
   fout.close();
   return 0;
}