//Normal

#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 prints(auto a){
  cout << a << " ";
}

void prints(){
  cout << " ";
}

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

void printl(){
  cout << endl;
}

void fix(int n){
  cout << fixed << setprecision(n);
}

int siz(string s){
  return (int)s.size();
}

int main(){
  int K, N, F; cin >> K >> N >> F;
  int sum = K*N;
  
  for(int i = 0; i < F; i++){
    int A; cin >> A;
    sum -= A;
  }
  
  printl(max(-1, sum));
  return 0;
}