#include<fstream>
using namespace std;
int  n,t1,t2,t3,i,a[1000][1000],j,m,s,suma;
ifstream fin("flip.in");
ofstream fout("flip.out");
int main(){
    fin >> n >> m;
    for (i = 1; i <= n; i++) {
        s = 0;
        for (j = 1; j <=  m; j++) {
            fin >> a[i][j];
            //fout <<a[i][j]<<" ";
            s = s + a[i][j];
            suma = suma + a[i][j];
            //fout <<s<<" ";
        }
        //fout <<s<<" ";
        if(s < 0){
           // fout <<"ok";
        for (j = 1; j <=  m; j++) {
            suma = suma -a [i][j] ;
            a[i][j] = a[i][j] * -1;
            //fout <<a[i][j]<<" ";
            suma = suma + a [i][j] ;

        }
        }
        //fout<< '\n';

    }
    fout <<suma;



}