#include <fstream>
#include <vector>

using std::vector;

int main() {
   std::ifstream in("ciclueuler.in");
   std::ofstream out("ciclueuler.out");
   int n, m;
   in >> n >> m;
   vector<vector<int>> graf(n);
   while(m--) {
      int u, v;
      in >> u >> v;
      u--; v--;
      graf[u].push_back(v);
      graf[v].push_back(u);
   }
   int nrOdd = 0;
   for(int i = 0; i < n; i++) {
      if(graf[i].size() % 2 == 1) {
         nrOdd++;
      }
      if(nrOdd > 2) {
         out << "-1";
         return 0;
      }
   }
}