#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <cmath>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <stdlib.h>
#include <stdio.h>
using namespace std;

#define pb         push_back
#define mp         make_pair
#define ll         long long
#define PI         acos(-1.0)
#define ALL(A)     (A).begin(), (A).end()
#define vsort(v)   sort(v.begin(),v.end())
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)

int main(){
  int K, N, F;
  cin >> K >> N >> F;
  int sum_A = 0, input;
  FOR(i, 0, F){
    cin >> input;
    sum_A += input;
  }
  int ans;
  if(sum_A > K * N) ans = -1;
  else ans = K * N - sum_A;

  cout << ans << endl;
}