#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define sz(n) (int)(n).size()
using namespace std;
typedef long long ll;
typedef vector<int> VI;

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