还是A+B

#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int main(){
    int A, B, K;
    while(~scanf("%d %d %d", &A, &B, &K) && A && B){
        int tmp = (int)pow(10, K);
        if(A % tmp == B % tmp){
            printf("-1\n");
        }else{
            printf("%d\n", A + B);
        }
    }
    return 0;
}