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