#include <iostream>
using namespace std;
int main(int argc, char* argv[])
{
	int K,N,F;
	cin>>K>>N>>F;
	int S=0;
	int A;
	for (int i=0;i<F;i++){
		cin>>A;
		S+=A;
	}
	int ans=K*N-S;
	if (ans>=0){
		cout<<ans<<endl;
	}else{
		cout<<-1<<endl;
	}
	return 0;
}