#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void printL(auto a){
  cout << a << endl;
}

void calc(int i, int x, vector<int> A){
  if(i == (int)A.size()){
    x = max(x, -1);
    printL(x);
    return;
  }
  calc(i+1, x-A[i], A);
}

int main(){
  int K, N, F; cin >> K >> N >> F;
  vector<int> A(F);
  for(int i = 0; i < F; i++) cin >> A[i];
  calc(0, K*N, A);
}