#include <bits/stdc++.h>
using namespace std;
int main( ){
   int K,N,F;
   cin >> K >> N >> F;
   vector<int> Family(F);
   for(int i = 0; i < F; i++){
       cin >> Family.at(i);
   }
   int age = 0;
   for(int i = 0; i < F; i++){
       age += Family.at(i);
   }
   if(age > K*N ) cout << -1 << endl;
   else cout << K*N - age << endl;
}