#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
long long modpow(long long a, long long n, long long m) {
  long long ans = 1;
  while (n) {
    if (n & 1) {
      ans = (ans * a) % m;
    }
    a = (a * a) % m;
    n >>= 1;
  }
  return ans;
}
long long combi(long long n, long long a) {
  long long ans = 1, ans1 = 1;
  for (long long i = n - a + 1; i <= n; i++) {
    ans *= i % MOD;
    ans %= MOD;
  }
  for (long long i = 2; i <= a; i++) ans1 = (ans1 * i) % MOD;
  ans1 = modpow(ans1, MOD - 2, MOD);
  return ((ans % MOD) * ans1) % MOD;
}
void dfs(string s, char mx, long long N) {
  if (s.length() == N) {
    cout << s.c_str() << endl;
  } else {
    for (char c = 'a'; c <= mx; c++) {
      dfs(s + c, ((c == mx) ? (char)(mx + 1) : mx), N);
    }
  }
}
int bfs(int a, int b, int h, int w, char tmp[][101]) {
  int ans[101][101] = {0};
  char c[101][101] = {0};
  queue<pair<int, int>> s;
  for (int i = 0; i <= h; i++) {
    for (int j = 0; j <= w; j++) {
      c[i][j] = tmp[i][j];
    }
  }
  s.push(make_pair(a, b));
  while (s.size() > 0) {
    pair<int, int> tmp = s.front();
    s.pop();
    c[tmp.first][tmp.second] = '#';
    for (int i = -1; i <= 1; i++) {
      for (int j = -1; j <= 1; j++) {
        if (tmp.first + i < 1 || tmp.first + i > h) {
          continue;
        }
        if (tmp.second + j < 1 || tmp.second + j > w) {
          continue;
        }
        if (i != 0 && j != 0) {
          continue;
        }
        if (i == 0 && j == 0) {
          continue;
        }
        if (c[tmp.first + i][tmp.second + j] == '#') {
          continue;
        }
        c[tmp.first + i][tmp.second + j] = '#';
        if (ans[tmp.first + i][tmp.second + j] == 0) {
          ans[tmp.first + i][tmp.second + j] = ans[tmp.first][tmp.second] + 1;
        } else {
          ans[tmp.first + i][tmp.second + j] =
              min(ans[tmp.first + i][tmp.second + j],
                  ans[tmp.first][tmp.second] + 1);
        }
        s.push(make_pair(tmp.first + i, tmp.second + j));
      }
    }
  }
  int asd = 0;
  for (int i = 1; i <= h; i++) {
    for (int j = 1; j <= w; j++) {
      asd = max(a, ans[i][j]);
    }
  }
  return asd;
}
int main() {
  long long n, M;
  cin >> n >> M;
  map<int, int> m;
  for (int i = 0; i < M; i++) {
    int s, c;
    cin >> s >> c;
    if (m[s] != 0 && m[s] != c) {
      cout << -1 << endl;
      return 0;
    }
    m[s] = c;
  }
  for (int i = 0; i < 1000; i++) {
    string tmp = to_string(i);
    bool f = true;
    for (auto i = m.begin(); i != m.end(); ++i) {
      const char a = tmp[i->first - 1];
      const char b = *to_string(i->second).c_str();
      if (i->first == 1 && i->second == 0) {
        f = false;
        break;
      }
      if (a != b) {
        f = false;
        break;
      }
    }
    if (f) {
      cout << i << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}
