#include <bits/stdc++.h>
#define PB push_back
#define MP make_pair
#define REP(i,n) for (int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define ALL(a) (a).begin(),(a).end()
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> P;
const ll MOD=1e9+7;
int main(){
      int k,n,f,sum=0;
      int a[100];
      cin>>k>>n>>f;
      REP(i,f){
            cin>>a[i];
            sum+=a[i];
      }
      if(k*n<sum)cout<<-1<<endl;
      else cout<<k*n-sum<<endl;
}