#include <bits/stdc++.h>
int main(void) {
  int n, m;
  scanf("%d %d", &n, &m);
  int i, x[n];
  for (i = 0; i < n; i++) {
    x[i] = 10;
  }
  int xxx, s, c;
  xxx = 1;
  for (i = 0; i < m; i++) {
    scanf("%d %d", &s, &c);
    if (x[s - 1] != 10 && x[s - 1] != c) {
      xxx = 0;
    } else {
      x[s - 1] = c;
    }
  }
  if (n != 1 && x[0] == 0) {
    xxx = 0;
  }
  if (xxx == 1) {
    if (x[0] == 10) {
      x[0] = 1;
    }
    for (i = 1; i < n; i++) {
      if (x[i] == 10) {
        x[i] = 0;
      }
    }
    double aaa = 0.0;
    for (i = 0; i < n; i++) {
      aaa += (double)x[i] * pow(10, n - i - 1);
    }
    int xx = (int)aaa;
    printf("%d\n", xx);
  } else {
    printf("-1\n");
  }
  return 0;
}
