#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
int dx[] = {1, 0, -1, 0, 0};
int dy[] = {0, 1, 0, -1, 0};
const int INT_INF = (int)(2e9);
const ll LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));
std::string pad(int num) {
  char buffer[4];
  std::snprintf(buffer, sizeof(buffer), "%03d", num);
  return buffer;
}
inline ll randint(ll a, ll b) {
  ll w = (_g() << 31LL) ^ _g();
  return a + w % (b - a + 1);
}
inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T>
inline T sign(T x) {
  return T(x > 0) - T(x < 0);
}
template <typename T, typename S>
inline ostream& operator<<(ostream& os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T, typename S>
inline ostream& operator<<(ostream& os, const map<T, S> p) {
  for (auto el : p) cout << "[" << el.first << "," << el.second << "] ";
  return os;
}
template <typename T>
inline ostream& operator<<(ostream& os, const vector<T>& v) {
  for (auto el : v) cout << el << " ";
  return os;
}
template <typename T>
inline T fetch() {
  T ret;
  cin >> ret;
  return ret;
}
template <typename T>
inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto& elem : ret) cin >> elem;
  return ret;
}
int N, M;
vector<int> s, c;
void input() {
  cin >> N >> M;
  for (int i = 0; i < M; i++) {
    int x, y;
    cin >> x >> y;
    s.push_back(x);
    c.push_back(y);
  }
}
map<int, set<int>> Num;
map<int, int> Num2;
int solve() {
  for (int i = 0; i < M; i++) Num[s[i]].insert(c[i]);
  for (int k = 1; k <= N; k++)
    if (Num[k].size() >= 2) {
      cout << -1 << endl;
      return 0;
    }
  for (int i = 0; i < M; i++) Num2[s[i]] = c[i];
  if (Num2[1] == 0) {
    cout << -1 << endl;
    return 0;
  }
  int ans = 0;
  if (N == 3) ans = Num2[1] * 100 + Num2[2] * 10 + Num2[3];
  if (N == 2) ans = Num2[1] * 10 + Num2[2];
  if (N == 1) ans = Num2[1];
  cout << ans << endl;
  return 0;
}
int main() {
  input();
  solve();
  return 0;
}
