/*
1229 還是A+B
Time Limit : 1000 ms  Memory Limit : 32768 K  Output Limit : 256 K

GUN C++
*/
#include <iostream.h>
#include <math.h>
using namespace std;

const int MAX=10000,KMAX=8;

int main()
{
    int a,b,k,ca,temp;
    while(cin>>a>>b>>k && (a!=0 || b!=0))
    {
        for(ca=0,temp=1;ca<k;ca++)
            temp*=10;
        if(a%temp==b%temp)
        {   cout<<-1<<endl;}
        else
        {   cout<<a+b<<endl;}
    }
    return 0;
    
}