#include <bits/stdc++.h>
using namespace std;

long long a,minv=1000003;
int main(void){
int a,b,c;
cin>>a>>b>>c;
int d;
int sum=0;
for (int i = 0; i < c; ++i) {
	cin>>d;
	sum=sum+d;
}
if(a*b-sum<0){
	cout<<-1<<endl;
}else{
	cout<<a*b-sum<<endl;
}

}