#include <bits/stdc++.h>
using namespace std;
int k,n,f,a,tot;
int main(){
//    freopen("aa.in","r",stdin);
//    freopen("aa.out","w",stdout);
    cin>>k>>n>>f;
    for(int i=0;i<f;++i){
        cin>>a;
        tot+=a;
    }
    if(k*n>=tot)
        cout<<k*n-tot;
    else
        cout<<-1;
    return 0;
}