#include <bits/stdc++.h>
void debug_out() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_out(Head h, Tail... t) {
  std::cerr << " " << h;
  debug_out(t...);
}
template <typename T1, typename T2>
std::ostream& operator<<(std::ostream& os, std::pair<T1, T2> pa) {
  return os << pa.first << " " << pa.second;
}
template <typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> vec) {
  for (std::size_t i = 0; i < vec.size(); i++)
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}
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);
}
long long pow_mod(long long a, long long b, long long mod = -1) {
  if (b == 0) return 1;
  if ((a == 0) || (mod != -1 && (a + mod) % mod == 0)) return 0;
  long long x = 1;
  while (b > 0) {
    if (b & 1) x = (mod != -1) ? (x * a) % mod : x * a;
    a = (mod != -1) ? (a * a) % mod : a * a;
    b >>= 1;
  }
  return x;
}
const long long MOD = 1e9 + 7;
using ll = long long;
using P = std::pair<long long, long long>;
ll gcd(ll a, ll b) {
  if (a % b == 0) return b;
  return gcd(b, a % b);
}
int main() {
  std::cin.tie(nullptr);
  std::ios::sync_with_stdio(false);
  ll n, m;
  std::cin >> n >> m;
  std::vector<ll> ans(n, -1);
  for (long long i = (0); (i) < (m); (i)++) {
    ll s, c;
    std::cin >> s >> c;
    s--;
    if (ans[s] != -1 && ans[s] != c) {
      std::cout << -1 << "\n";
      return 0;
    }
    ans[s] = c;
  }
  if (n == 1) {
    if (ans[0] == -1) {
      std::cout << 0 << "\n";
      return 0;
    }
    std::cout << ans[0] << "\n";
    return 0;
  }
  if (ans[0] == 0) {
    std::cout << -1 << "\n";
    return 0;
  }
  for (long long i = (0); (i) < (n); (i)++) {
    if (ans[i] != -1)
      std::cout << ans[i];
    else
      std::cout << 0;
  }
  std::cout << "\n";
  return 0;
}
