#include <bits/stdc++.h>
using namespace std;
int n, m, a = 0;
pair<int, int> sc[5];
int main() {
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int s, c;
    cin >> s >> c;
    sc[i].first = n - s;
    sc[i].second = c;
  }
  sort(sc, sc + m);
  for (int i = 0; i < m; i++) {
    int s, c, t = 1, b;
    s = sc[i].first;
    c = sc[i].second;
    for (int j = 0; j < s; j++) t *= 10;
    b = a % (10 * t) / t;
    if ((b != 0 && b != c) || (s > 0 && c == 0)) {
      a = -1;
      break;
    } else if (b == 0)
      a += c * t;
  }
  cout << a << endl;
  return 0;
}
