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

#define step(i, s, n, d) for(int i=s; i<n; i+=d)
#define FOR(i,s,n) step(i,s,n,1)
#define rep(i,n) FOR(i,0,n)
#define gets(x) x; cin >> x;
#define puts(x) x; cout << x << endl;
#define list_input(x, n) x[n]; rep(i,n){ gets(x[i]);}
#define ll long long

int main(){
  int k, n, f, result;
  cin >> k >> n >> f;
  result = k*n;
  int a[f];
  list_input(a, f)
  rep(i,f){
    result -= a[i];
  }
  result = result >= 0? result : -1;
  cout << result << endl;
}