#include <bits/stdc++.h>
using namespace std;
vector<int> g[9];
int main() {
  int n, m;
  cin >> n >> m;
  int c[9];
  memset(c, 9, sizeof(c));
  if (m == 0) {
    cout << 0 << endl;
    exit(0);
  }
  for (int i = 0; i < m; i++) {
    int x, y;
    cin >> x >> y;
    c[x] = min(c[x], y);
    g[x].push_back(y);
  }
  if (g[1].size() == 0) c[1] = 0;
  if (c[1] == 0 && n > 1) {
    cout << -1 << endl;
    exit(0);
  }
  int sum = 0;
  for (int i = 1; i <= n; i++) {
    if (g[i].size()) {
      sum = sum * 10 + c[i];
    } else
      sum = sum * pow(10, i - 1);
  }
  cout << sum << endl;
}
