#include <bits/stdc++.h>

#define foreach(i, v) for(__typeof((v).begin()) i = (v).begin(); i != (v).end(); ++ i)
#define BETWEEN(a, x, b) (a<=x&&x<b)

using namespace std;
typedef long long ll;

int main(int argc, char *argv[]) {
  ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);

  int k,n,f;
  cin>>k>>n>>f;
  int t = k * n;
  for(int i=0;i<f;i++){
    int p;cin>>p;t -= p;
  }
  cout<<(t >= 0 ? t : -1)<<endl;

  return 0;
}