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