#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
int main() {
  {
    ios::sync_with_stdio(0);
    cin.tie(nullptr);
    cout.tie(nullptr);
  }
  int n, m;
  cin >> n >> m;
  int a[n + 1], s, t, fl = 0;
  memset(a, -1, sizeof(a));
  for (int i = 1; i <= (int)(m); ++i) {
    cin >> s >> t;
    if (a[s] == -1)
      a[s] = t;
    else if (a[s] != t) {
      fl = 1;
    }
  }
  if (fl == 1) {
    cout << -1;
    return 0;
  }
  if (n == 1) {
    if (a[s] == -1) {
      cout << 0;
      return 0;
    } else {
      cout << a[1];
      return 0;
    }
  }
  if (a[1] == 0) {
    cout << -1;
    return 0;
  }
  if (a[1] == -1) a[1] = 1;
  string str = to_string(a[1]);
  for (int i = 2; i <= n; i++) {
    if (a[i] == -1)
      str += '0';
    else
      str += a[i] + '0';
  }
  cout << str;
}
