N, M = map(int, input().split())

S = []
C = []
for i in range(M):
    s, c = map(int, input().split())
    S.append(s)
    C.append(c)


try:
    Z = [[] for i in range(M)]
    for j in range(M):
        for i in range(10**(N-1)+1, 10**N):
            if S[j] > M : 
                break
            i = str(i)
            if i[S[j]-1] == str(C[j]):
                Z[j].append(int(i))

    ZZ = Z[0]
    for j in range(1, M):
        set_zz = set(ZZ)
        set_z = set(Z[j])
        ZZ = set_zz & set_z

    print(min(ZZ))
except:
    print(-1)