N,M = list(map(int,input().split()))
l = [list(map(int,input().split())) for i in range(M)]
L = sorted(l)
if N == 1 and L[0][1] == 0:
    ans = 0
elif N != 1 and L[0][1] == 0:
    ans = -1
else:
    for n in range(M-1):
        if (L[n][0] == L[n+1][0]) and (L[n][1] != L[n+1][1]):
            ans = -1
            break
        else:
            s = sorted(list(set(list(map(tuple,L)))))
            m = len(s)
            if s[0][0] != 1:
                ans = 10**(M-1)
            else:
                ans = 0
            for n in range(m):
                ans += s[n][1]*(10**(M-s[n][0]))
print(ans)