#include <bits/stdc++.h>
using namespace std;

signed main(){
  int K, N, F; cin >> K >> N >> F;
  vector< int > A( F );
  for( int i = 0; i < F; ++i )
    cin >> A[ i ];
  int cnt = K * N;
  for( int i = 0; i < F; ++i )
    cnt -= A[ i ];
  if( cnt < 0 )
    cnt = -1;
  cout << cnt << endl;
  return 0;
}