#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define pb push_back
#define mp make_pair
#define rep(i,n) for(int i=0;i<(n);++i)
const int mod=1000000007;

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int k,n,f;cin >> k >> n >> f;
	int m=k*n;
	vector<int> a(f);
	rep(i,f) cin >> a.at(i);
	rep(i,f){
		m-=a.at(i);
		if(m<0){
			cout << -1 << endl;
			return 0;
		}
	}
	cout << m << endl;
}