#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  int k,n,f;cin>>k>>n>>f;
  int cnt=0,hoge;
  rep(i,f){
    cin>>hoge;
    cnt+=hoge;
  }
  cout<<(k*n>cnt?k*n-cnt:-1)<<endl;
}