#include<iostream>

using namespace std;

int main(){

	int N,K,F,A;
	int c;

	cin>>N>>K>>F;

	c=N*K;
	for(int i=0;i<F;i++){
		cin>>A;
		c-=A;
	}

	if(c<0) cout<<"-1"<<endl;
	else cout<<c<<endl;
}