#include <iostream>
using namespace std;
int main()
{
    int k,n,f,t,sum=0;
    cin>>k>>n>>f;
    for(int i=0;i<f;i++){
        cin>>t;
        sum+=t;
    }
    t=k*n-sum;
    if(t<0)t=-1;
    cout<<t<<endl;
    return 0;
}