#include <iostream>
#include <vector>
using namespace std;

int main() {
	int k,n,f;
	vector<int> a;
	
	scanf("%d %d %d", &k, &n, &f);
	
	int t;
	for(int i = 0; i < f; i++) {
		scanf("%d", &t);
		a.push_back(t);
	}
	
	int beans = k * n;
	
	for (int i = 0; i < a.size(); i++) {
		beans -= a[i];
	}
	
	if (beans < 0) {
		printf("-1\n");
	}
	else {
		printf("%d\n", beans);
	}
	
	return 0;
}