#define ll long long
#define ffor(i,a,b) for (int i=(a);i<(b);i++)
#define rfor(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define rep(i,n) for (int i=0;i<(n);i++)
#define rrep(i,n) for (int i=(n)-1;i>=0;i--)
#include<iostream>
#include<iomanip>
#include<algorithm>
#include<cmath>
#include<string>
#include<stack>
#include<queue>
#define SIZE 100001
#define MOD 1000000007
#define INF 100000000
using namespace std;

int main(){
  int k,n,f;
  int a[100];
  cin >> k >> n >> f;
  rep(i,f) cin >> a[i];

  int sum = k*n;

  rep(i,f) sum -= a[i];

  cout << ((sum<0)? -1 :sum ) <<endl;

}