#include <fstream>

#define MAX_WIDTH 500
#define MAX_HEIGHT 500

int Table[MAX_WIDTH][MAX_HEIGHT];

int N,M;
int s;

using namespace std;


int main()
{
	ofstream out;
	ifstream in;
	in.open("flip.in");
	out.open("flip.out");
	if(in.is_open())
	{
		while(in.good())
		{
		in >> N;in>>M;
		for(int i = 0; i < N; i++)
		{
			for(int j = 0; j < M; j++)
			{
				in >>Table[i][j];
				//cout <<Table[N][M];
			}
		}
		}
	}
	s = 0;
	for(int i = 0; i < N; i++)
	{
		for(int j = 0; j < M; j++)
		{
			if(j < M)
			{
				if(Table[i][j] + Table[i][j+1] < -(Table[i][j] + Table[i][j+1]))
				{
					Table[i][j] = -Table[i][j];
				}
			}
			s = s + Table[i][j];
		}
	}
	out << s;
	in.close();
	out.close();
	return 0;
}