#include <bits/stdc++.h>
int main() {
  int n, m, i, j, k;
  int a = 10, b = 10, c = 10;
  int z = 0;
  int s[10][2];
  scanf("%d %d", &n, &m);
  if (m != 0) {
    for (i = 0; i < m; i++) {
      scanf("%d %d", &s[i][0], &s[i][1]);
    }
  }
  if (m != 0) {
    for (i = 0; i < m; i++) {
      if (s[i][0] == 1) {
        if (s[i][1] == 0 && n != 1) {
          printf("-1");
          return 0;
        }
        if (a == 10) {
          a = s[i][1];
          z = z + a * 100;
        } else if (s[i][1] != a) {
          printf("-1");
          return 0;
        }
      }
      if (s[i][0] == 2) {
        if (b == 10) {
          b = s[i][1];
          z = z + b * 10;
        } else if (s[i][1] != b) {
          printf("-1");
          return 0;
        }
      }
      if (s[i][0] == 3) {
        if (c == 10) {
          c = s[i][1];
          z = z + c;
        } else if (s[i][1] != c) {
          printf("-1");
          return 0;
        }
      }
    }
  } else {
    printf("-1");
    return 0;
  }
  printf("%d", z);
  return 0;
}
