while(1):
    try:
        num=list(map(int,input().split()))
        a=num[0]
        b=num[1]
        c=num[2]
        if(a==0 and b==0):
            break
        count=10**c
        if(a%count==b%count):
            print("-1")
            continue
        else:
            print(a+b)

    except:
        break