#include <iostream>
#include <vector>
#include <algorithm>
#define REP(i,n) for(int i=0;i<(n);++i)
#define SORT(a) sort((a).begin(),(a).end())
#define PB push_back
using namespace std;
typedef vector<int> VI;

int main(){
  int K, N, F;
  cin >> K >> N >> F;
  int r = K * N;
  REP(i,F){
    int A;
    cin >> A;
    r -= A;
  }
  cout << (r < 0 ? -1 : r) << endl;
}