#include <iostream>
#include <vector>

using namespace std;

int main(){
	int k,n,f,beans;
	cin >>k>>n>>f;
	vector<int> year(f);
	for(int i=0;i<f;i++){
		cin >> year[i];
	}
	beans = k*n;
	for(int i=0;i<f;i++){
		beans -= year[i];
	}
	if(beans<0){
		beans = -1;
	}
	cout << beans <<endl;
}