#include <bits/stdc++.h>
using namespace std;
template <class T1, class T2>
bool chmin(T1 &a, T2 b) {
  return b < a && (a = b, true);
}
template <class T1, class T2>
bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using P = pair<ll, ll>;
ll ugauss(ll a, ll b) {
  if (!a) return 0;
  if (a > 0 ^ b > 0)
    return a / b;
  else
    return (a + (a > 0 ? -1 : 1)) / b + 1;
}
ll lgauss(ll a, ll b) {
  if (!a) return 0;
  if (a > 0 ^ b > 0)
    return (a + (a > 0 ? -1 : 1)) / b - 1;
  else
    return a / b;
}
template <typename T, typename U>
ostream &operator<<(ostream &o, const pair<T, U> &v) {
  o << "(" << v.first << ", " << v.second << ")";
  return o;
}
template <size_t...>
struct seq {};
template <size_t N, size_t... Is>
struct gen_seq : gen_seq<N - 1, N - 1, Is...> {};
template <size_t... Is>
struct gen_seq<0, Is...> : seq<Is...> {};
template <class Ch, class Tr, class Tuple, size_t... Is>
void print_tuple(basic_ostream<Ch, Tr> &os, Tuple const &t, seq<Is...>) {
  using s = int[];
  (void)s{0, (void(os << (Is == 0 ? "" : ", ") << get<Is>(t)), 0)...};
}
template <class Ch, class Tr, class... Args>
auto operator<<(basic_ostream<Ch, Tr> &os, tuple<Args...> const &t)
    -> basic_ostream<Ch, Tr> & {
  os << "(";
  print_tuple(os, t, gen_seq<sizeof...(Args)>());
  return os << ")";
}
ostream &operator<<(ostream &o, const vvll &v) {
  for (long long i = 0; i < (long long)(v.size()); i++) {
    for (long long j = 0; j < (long long)(v[i].size()); j++)
      o << v[i][j] << " ";
    o << endl;
  }
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const vector<T> &v) {
  o << '[';
  for (long long i = 0; i < (long long)(v.size()); i++)
    o << v[i] << (i != v.size() - 1 ? ", " : "");
  o << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const deque<T> &v) {
  o << '[';
  for (long long i = 0; i < (long long)(v.size()); i++)
    o << v[i] << (i != v.size() - 1 ? ", " : "");
  o << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const set<T> &m) {
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it << (next(it) != m.end() ? ", " : "");
  o << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const unordered_set<T> &m) {
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it << (next(it) != m.end() ? ", " : "");
  o << "]";
  return o;
}
template <typename T, typename U>
ostream &operator<<(ostream &o, const map<T, U> &m) {
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it << (next(it) != m.end() ? ", " : "");
  o << "]";
  return o;
}
template <typename T, typename U, typename V>
ostream &operator<<(ostream &o, const unordered_map<T, U, V> &m) {
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++) o << *it;
  o << "]";
  return o;
}
vector<int> range(const int x, const int y) {
  vector<int> v(y - x + 1);
  iota(v.begin(), v.end(), x);
  return v;
}
template <typename T>
istream &operator>>(istream &i, vector<T> &o) {
  for (long long j = 0; j < (long long)(o.size()); j++) i >> o[j];
  return i;
}
template <typename T, typename S, typename U>
ostream &operator<<(ostream &o, const priority_queue<T, S, U> &v) {
  auto tmp = v;
  while (tmp.size()) {
    auto x = tmp.top();
    tmp.pop();
    o << x << " ";
  }
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const queue<T> &v) {
  auto tmp = v;
  while (tmp.size()) {
    auto x = tmp.front();
    tmp.pop();
    o << x << " ";
  }
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const stack<T> &v) {
  auto tmp = v;
  while (tmp.size()) {
    auto x = tmp.top();
    tmp.pop();
    o << x << " ";
  }
  return o;
}
template <typename T>
unordered_map<T, ll> counter(vector<T> vec) {
  unordered_map<T, ll> ret;
  for (auto &&x : vec) ret[x]++;
  return ret;
};
void vizGraph(vvll &g, int mode = 0, string filename = "out.png") {
  ofstream ofs("./out.dot");
  ofs << "digraph graph_name {" << endl;
  set<P> memo;
  for (long long i = 0; i < (long long)(g.size()); i++)
    for (long long j = 0; j < (long long)(g[i].size()); j++) {
      if (mode && (memo.count(P(i, g[i][j])) || memo.count(P(g[i][j], i))))
        continue;
      memo.insert(P(i, g[i][j]));
      ofs << "    " << i << " -> " << g[i][j]
          << (mode ? " [arrowhead = none]" : "") << endl;
    }
  ofs << "}" << endl;
  ofs.close();
  system(((string) "dot -T png out.dot >" + filename).c_str());
}
struct timeval start;
double sec() {
  struct timeval tv;
  gettimeofday(&tv, NULL);
  return (tv.tv_sec - start.tv_sec) + (tv.tv_usec - start.tv_usec) * 1e-6;
}
size_t random_seed;
struct init_ {
  init_() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    gettimeofday(&start, NULL);
    struct timeval myTime;
    struct tm *time_st;
    gettimeofday(&myTime, NULL);
    time_st = localtime(&myTime.tv_sec);
    srand(myTime.tv_usec);
    random_seed = RAND_MAX / 2 + rand() / 2;
  }
} init__;
int main(void) {
  ll n, m;
  cin >> n >> m;
  vll ans(n);
  vll count(n);
  for (long long i = 0; i < (long long)(m); i++) {
    ll s = 0;
    ll c = 0;
    cin >> s >> c;
    if (count[s - 1] == 0) {
      ans[s - 1] = c;
      count[s - 1]++;
    } else {
      if (ans[s - 1] == c) continue;
      if (ans[s - 1] != c) {
        cout << "-1" << endl;
        return 0;
      }
    }
  }
  if (n == 1) cout << ans[0] << endl;
  if (n == 2) {
    if (ans[0] == 0) {
      cout << "-1" << endl;
      return 0;
    } else
      cout << ans[0] << ans[1] << endl;
  }
  if (n == 3) {
    if (ans[0] == 0) {
      cout << "-1" << endl;
      return 0;
    } else
      cout << ans[0] << ans[1] << ans[2] << endl;
  }
  return 0;
}
