#include<bits/stdc++.h>
using namespace std;
int main(){
    int k,n,f,diff,ans;
    cin>>k>>n>>f;
    vector<int> a(f);
    for(int i=0;i<f;++i)cin>>a[i];
    diff=k*n-accumulate(a.begin(),a.end(),0);
    ans=(diff>=0?diff:-1);
    cout<<ans<<endl;
}