#include"bits/stdc++.h"
using namespace std;
using ll = long long;
using ld = long double;
#define rep(i,m,n) for(ll i=(ll)m;i<(ll)n;i++)
#define Endl endl
#define pr(i,j) make_pair(i,j) 
const ll mod = 998244353;
const ll inf = 5e18;
const ld pi = 3.14159265358979;
int main() {
	ll k, n, f;
	cin >> k >> n >> f;
	vector<ll>a(f);
	ll sum = 0;
	rep(i, 0, f) {
		cin >> a[i];
		sum += a[i];
	}
	cout << max(-1LL, k * n - sum)<<endl;
}