#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, l[10] = {0}, r[10] = {0};
  scanf("%d%d", &n, &m);
  for (int i = 0; i < m; i++) scanf("%d%d", &l[i], &r[i]);
  int ret[4];
  memset(ret, -1, sizeof(ret));
  int flag = 1;
  for (int i = 0; i < m; i++) {
    if (ret[l[i]] < 0 || ret[l[i]] == r[i])
      ret[l[i]] = r[i];
    else {
      flag = 0;
      break;
    }
  }
  if (flag) {
    if (n == 1) {
      if (ret[1] == -1) ret[1] = 0;
      printf("%d\n", ret[1]);
    } else if (n == 2) {
      if (ret[1] > 0) {
        if (ret[2] == -1) ret[2] = 0;
        printf("%d%d\n", ret[1], ret[2]);
      } else
        printf("-1\n");
    } else {
      if (ret[1] > 0) {
        if (ret[2] == -1) ret[2] = 0;
        if (ret[3] == -1) ret[3] = 0;
        printf("%d%d%d\n", ret[1], ret[2], ret[3]);
      } else
        printf("-1\n");
    }
  } else
    printf("-1\n");
  return 0;
}
