#include <bits/stdc++.h>
#define N 200007

using namespace std;

ifstream fin("2sat.in");
ofstream fout("2sat.out");

int n,ncomp,m,nn;
vector<int> g[N];
vector<int> gt[N];
vector<int> comp[N];
vector <bool>viz(N,0);
vector <int>st(N,0);
/// not(x) = x + TX

int negat(int x)
{
    if( x<0 )
    {
        return -x;
    }
    if( x>n )
        return x-n;
    return x+n;
}

int adevarat(int x)
{
    if( x<0 )
    {
        return -x+n;
    }
    if( x>n )
        return -(x-n);
    return x;
}

void Citire()
{
    fin >> n >> m;nn=2*n;
    for(int i=1;i<=m;i++)
    {
        int x,y;
        fin >> x >> y;
//cout << i << "\n";
        g[ negat(x) ].push_back(adevarat(y));
//cout << negat(x)  << " "  << adevarat(y) << "\n";
        g[ negat(y) ].push_back(adevarat(x));
//cout << negat(y) << " " << adevarat(x) << "\n";
        gt[ adevarat(y) ].push_back(negat(x));
        gt[ adevarat(x) ].push_back(negat(y));
    }
    fin.close();
}


void dfs1(int x)
{
    viz[x]=1;
    for(auto y:g[x])
        if( !viz[y] )
            dfs1(y);
    st[nn--]=x;
}
void dfs2(int x)
{
    viz[x]=0;
    for(auto y:gt[x])
        if( viz[y] )
            dfs2(y);
    comp[ncomp].push_back(x);
}


void Kosaraju()
{
    for(int i=1;i<=2*n;i++)
        if( !viz[i] )
            dfs1(i);
    for(int i=1;i<=2*n;i++)
        if( viz[ st[i] ] )
        {
            ncomp++;
            dfs2( st[i] );
        }
    // das good
}

bool Exista()
{
    for(int i=1;i<=ncomp;i++)
    {
        set<int> bucket;
        int sz=comp[i].size();
        for(int j=0;j<sz;j++)
        {
            if( bucket.count( adevarat(comp[i][j]) ) )return 0;
            bucket.insert( adevarat(comp[i][j]) ) ;
        }
    }
    return 1;
}


int main()
{
    Citire();
    Kosaraju();
    if( !Exista() )fout << "-1\n";
/// ... .. .. ... . .. etc

    fout.close();
    return 0;
}