using namespace std;
#include <bits/stdc++.h>
int main(){
    int k,n,f;
    cin>>k>>n>>f;
    std::vector<int>a(f) ;
    long long sum=0;
    for(int i=0;i<f;i++){cin>>a.at(i);sum+=a.at(i);}
    if(k*n>=sum)cout<<k*n-sum<<endl;
    else{
        cout<<-1<<endl;
    }
    return 0;
}