#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("flip.in");
ofstream g("flip.out");
int n,m;
long a[17][17],s;
void citire()
{int i,j;
 f>>n>>m;
 for(i=1;i<=n;i++)
  for(j=1;j<=m;j++)f>>a[i][j];
}
long v[34],p=0;
bool cresc(long b,long c)
{return b<c;
}
void sortare(long&p)
{int i,j;
 for(i=1;i<=n;i++)
     for(j=1;j<=m;j++)v[++p]=a[i][j];
sort(v+1,v+p+1,cresc);
}
int main()
{citire();
sortare(p);
int ok=0;
int i;
for(i=1;i<=p;i++)
{if(v[i]<0&&v[i+1]<0)ok=0;
 if(v[i]<0&&v[i+1]>=0)
 {ok=1;
  i=i+1;
   break;
 }
}
for(int j=i;j<=p;j++)s+=v[j];
g<<s;
    return 0;
}