#include <bits/stdc++.h>
#define INF 990000000
//37
using namespace std;

int main() {
int k,n,f;
cin>>k>>n>>f;

int a,res=0;
for(int i=0;i<f;i++){
cin>>a;
res+=a;
}
if(k*n-res<0)
cout <<-1<<endl;
else
cout <<k*n-res<<endl;


}