#include <iostream>
//#include<atcoder/all>
#include <numeric>
#include <cmath>
#include <limits>
#include <stdio.h>
#include <iomanip>
#include <string> 
#include <vector>
#include <algorithm>
#include <utility> 
#include <tuple> 
#include <cstdint> 
#include <cstdio> 
#include <map> 
#include <queue>
#include <set> 
#include <stack> 
#include <deque> 
#include <unordered_map> 
#include <unordered_set>
#include <bitset> 
#include <cctype>
//using namespace atcoder;
using namespace std;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<pair<ll, ll>>>;
vector<bool> seen;
bool C = true;
vector<ll> dist;
ll mod =1e9+7;
int op(int a, int b) {
	return min(a, b);
}
int e() {
	return int(1e9);
}

int main() {
	ll K,N,F;
	cin >> K>>N>>F;
	ll S = 0;
	rep(f, F) {
		ll A;
		cin >> A;
		S += A;
	}
	if (K * N >= S)cout << K * N - S << endl;
	else cout << -1 << endl;
	
}