#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>
#include <cstring>
#include <cctype>
#include <cmath>
#include <stack>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <list>
#include <stdio.h>
#include <string.h>
#include <cstdlib>
#include <math.h>
#include <bitset>
#include <iterator>
#include <iomanip>
#include <sstream>
#define INF 2000000000
using namespace std;
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
#define sym cout<<"---------"<<endl;
#define ll long long
#define PL pair<long long, long long>
#define P pair<int,int>
#define mk make_pair
#define en endl
#define WHITE 0
#define BLACK 2
#define GRAY 1
#define RE return 0
#define int ll

int peo[105];

signed main(){
  int k,n,f;
  cin>>k>>n>>f;
  int ans=0;
  for(int i=0; i<f; i++){
    cin>>peo[i];
    ans+=peo[i];
  }
  
  if(k*n>=ans){
    cout<<k*n-ans<<en;
    RE;
  }else{
    cout<<-1<<en;
    RE;
  }
}