hdu1229

/**/
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cctype>
#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <vector>
#include <string>
#include <stack>
#include <queue>

typedef long long LL;
using namespace std;

int a, b, k;

int main()
{
	//freopen("in.txt", "r", stdin);
	//freopen("out.txt", "w", stdout);

	while(scanf("%d %d %d", &a, &b, &k) == 3){
		if(!a && !b) break;
		int sum = a + b;
		int cnt = 0;
		int flag = 0;
		while(a || b){
			int t1 = a % 10;
			a /= 10;
			int t2 = b % 10;
			b /= 10;
			if(t1 == t2){
				cnt++;
			}else{
				flag = 1;
				break;
			}
		}
		if(cnt >= k || (!a && !b && !flag)){
			printf("%d\n", -1);
		}else{
			printf("%d\n", sum);
		}
	}

	return 0;
}
/**/