#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  int s[m], c[m];
  vector<int> ans(n);
  for (int i = 0; i < (m); i++) {
    cin >> s[i] >> c[i];
  }
  for (int i = 0; i < m - 1; i++) {
    for (int j = i + 1; j < m; j++) {
      if (s[i] == s[j] && c[i] != c[j]) {
        cout << "-1" << endl;
        return 0;
      }
    }
  }
  for (int i = 0; i < (m); i++) ans[s[i] - 1] = c[i];
  while (ans.size() > 0 && ans[0] == 0) ans.erase(ans.begin());
  if (n != ans.size())
    cout << "-1";
  else
    for (int i = 0; i < (n); i++) cout << ans[i];
  cout << endl;
  return 0;
}
