N,M= list(map(int, input().split()))
P = [list(map(int,input().split())) for i in range(M)]
Q = list(map(list, set(map(tuple, P))))
C = []
S = []
n = N -1
sum1 = 0

for s in range(len(Q)):
    C.append(Q[s][0])
for t in range(len(Q)):
    S.append(Q[t][1])
D = list(set(C))
if len(D) <len(S) :
    print("-1")
elif N < max(C):
    print("-1")
else:
    if N == 1 and M == 1 and Q[0][1] == 0:
        print("0")

    else:
        for u in range(len(Q)):
            sum1 = sum1 + Q[u][1]*(10 ** (N - Q[u][0]))
        if sum1 >= (10 ** n):
            print(sum1)
        else:
            if D.count(1) >= 1:
                unko = C.index(1)
                if Q[unko][1] == 0:
                    print("-1")
                else:
                    print(sum1 + 10 ** n)
            else:
                print(sum1 + 10 ** n)
