#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <queue>
#include <algorithm>
using namespace std;

#define REP(i,n) for(int i=0; i<n; i++)
#define FOR(i,a,b) for(int i=a; i<=b; i++)
#define FORR(i,a,b) for (int i=a; i>=b; i--)

typedef long long ll;

int main(void){
	int k,n,f;
	cin >> k >> n >> f;
	int ans = k*n;
	REP(i,f){
		int a;
		cin >> a;
		ans -= a;
	}
	if(ans>=0)
		cout << ans << endl;
	else 
		cout << -1 << endl;
	return 0;
}