#include <bits/stdc++.h>
using namespace std;
template <typename T>
void DEBUG(T e) {
  if (true == false) return;
  std::cout << e << std::endl;
}
template <typename T>
void DEBUG(const std::vector<T>& v) {
  if (true == false) return;
  for (const auto& e : v) {
    std::cout << e << " ";
  }
  std::cout << std::endl;
}
template <typename T>
void DEBUG(const std::vector<std::vector<T> >& vv) {
  if (true == false) return;
  for (const auto& v : vv) {
    DEBUG(v);
  }
}
template <class T, class X>
void DEBUG(X d, T e) {
  if (true == false) return;
  std::cout << d << " ";
  DEBUG(e);
};
template <class T>
void corner(bool flg, T hoge) {
  if (flg) {
    cout << hoge << endl;
    exit(0);
  }
}
template <typename T1, typename T2>
inline bool chmax(T1& a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2>
inline bool chmin(T1& a, T2 b) {
  return a > b && (a = b, true);
}
void solve(void) {
  long long N, M;
  cin >> N >> M;
  vector<long long> vec(N, -1);
  for (long long i = 0; i < M; i++) {
    long long A, B;
    cin >> A >> B;
    A--;
    if (vec[A] == -1 || vec[A] == B) {
      vec[A] = B;
    } else {
      cout << -1 << endl;
      return;
    }
  }
  if (vec[N - 1] <= 0) {
    cout << -1 << endl;
    return;
  } else {
    for (long long i = 0; i < vec.size(); i++) {
      if (vec[i] == -1) vec[i] = 0;
      cout << vec[i];
    }
    cout << endl;
  }
  return;
}
int32_t main(int32_t argc, const char* argv[]) {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);
  std::cout << std::fixed;
  std::cout << std::setprecision(9);
  solve();
  return 0;
}
