n,m = [int(x) for x in input().split()]
sc = {}
out = False
for _ in range(m):
    s,c = [x for x in input().split()]
    if s in sc.keys():
        if sc[s] != c:
            print("-1")
            out = True
        else:
            sc[s] = c
    else:
        sc[s] = c
        

if not out:
    if n == 1:
        print(sc["1"])
    elif n == 2:
        if "1" in sc.keys():
            if sc["1"] != "0":
                if "2" in sc.keys():
                    print(sc["1"],sc["2"],sep="")
                else:
                    sc["2"] = "0"
                    print(sc["1"],sc["2"],sep="")
            else:
                print("-1")
        else:
            sc["1"] = "1"
            print(sc["1"],sc["2"],sep="")
    elif n == 3:
        if "1" in sc.keys():
            if sc["1"] != "0":
                if "2" in sc.keys():
                    if "2" in sc.keys():
                        print(sc["1"],sc["2"],sc["3"],sep="")
                    else:
                        sc["3"] = "0"
                        print(sc["1"],sc["2"],sc["3"],sep="")
                else:
                    sc["2"] = "0"
                    if "3" in sc.keys():
                        print(sc["1"],sc["2"],sc["3"],sep="")
                    else:
                        sc["3"] = "0"
                        print(sc["1"],sc["2"],sc["3"],sep="")
            else:
                print("-1")
        else:
            sc["1"] = "1"
            if not "2" in sc.keys():
                sc["2"] = "0"
            if not "3" in sc.keys():
                sc["3"] = "0"
            print(sc["1"],sc["2"],sc["3"],sep="")