#include <iostream>
#include <stdio.h>
#include <vector>

#define rep(i,n) for(int i = 0; i < n; i++)

typedef long long ll;

using namespace std;

const int INF = 1e9;
const int MOD = 1e9+7;
const double PI=3.14159265358979323846;

int main(){
  int k,n,f,ans,sum=0;
  cin >> k >> n >> f;
  rep(i,f){
    int a;
    cin >> a;
    sum+=a;
  }
  ans = k*n-sum;
  if(ans>=0)cout << ans << endl;
  else cout << -1 << endl;
}