#include <bits/stdc++.h>
using namespace std;
#define REP(i,N) for(i=0;i<N;i++)
typedef long long ll;
typedef pair<int, int> P;
typedef struct{
  int first;
  int second;
  int third;
}T;

//昇順
bool comp_Se(T& l, T& r){
  return l.second < r.second;
}

int main(void){
  int K,N,F;
  cin >> K >> N >> F;
  int i,x=0,sum = K*N;
  REP(i,F){
    cin >> x;
    sum -= x;
  }
  if(sum < 0) cout << -1 << endl;
  else cout << sum << endl;
  return 0;
}