#input
N, M = map(int, input().split())
s = [0] * M
c = [0] * M
for i in range(M):
    s[i], c[i] = map(int, input().split())

#output
from collections import Counter
dp = [-1] * N
for i in range(M):
    if dp[s[i]-1] == -1:
        dp[s[i]-1] = c[i]
    elif c[i] != dp[s[i]-1]:
        print(-1)
        exit()

if N == 1:
    print(dp[0])
else:
    if dp[0] == 0:
        print(-1)
    else:
        for i in range(N):
            if dp[i] == -1:
                print(0, end = "")
            else:
                print(dp[i])