#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <limits>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <complex>

using namespace std;

#define INF (INT_MAX/3)
#define PI (2*acos(0.0))
#define EPS (1e-8)

typedef long long ll;
typedef unsigned long long ull;

int main(){
  ios_base::sync_with_stdio(0);
  int K, N, F, A, sum = 0, r;
  cin >> K >> N >> F;
  for(int i = 0; i < F; i++){
    cin >> A;
    sum += A;
  }
  r = K * N - sum;
  if(r >= 0) cout << r << endl;
  else cout << -1 << endl;
  return 0;
}