#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
constexpr ll INF = 9e18;
constexpr int inf = 1e9;
const double INFD = 1e100;
const ll mod = 1000000007;
const double PI = 3.1415926535897932384626433832795028841971;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
};
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
};
template <typename T>
inline string toString(const T& a) {
  ostringstream oss;
  oss << a;
  return oss.str();
};
int main() {
  int N, M;
  cin >> N >> M;
  vector<int> S(N, -1);
  for (int i = 0; i < M; i++) {
    int s, c;
    cin >> s >> c;
    s--;
    if (S[s] == -1) {
      S[s] = c;
    } else if (S[s] != c) {
      cout << -1 << endl;
      return 0;
    }
  }
  int ans = -1;
  for (int i = 0; i < N; i++) {
    if (ans > 0) ans *= 10;
    if (S[i] <= 0) continue;
    if (ans == -1) ans = 0;
    ans += S[i];
  }
  cout << ans << endl;
  return 0;
}
