#include <bits/stdc++.h>
#define rep(i,a,b) for(int i = (a); i < (b); i++)
#define all(A) (A).begin(),(A).end()
#define MOD 1000000007
using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;

int main() {
	int k,n,f; cin >> k >> n >> f;
	vector<int> A(f); for(int& a : A) cin >> a;
	int ans = k*n;
	rep(i,0,f) ans -= A[i];
	cout << (ans < 0 ? -1 : ans);
}