#include <iostream>
#include <vector>
#include <climits>
#include <algorithm>
#include <cmath>
#include <map>
#include <set>
#include <string>
#include <bitset>
#include <utility>
#include <numeric>
#include <queue>
#include <stack>

using ll = long long;
using namespace std;

constexpr int MOD = 1e9 + 7;
constexpr ll MOD_LL = ll(1e9 + 7);

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