/*
これを入れて実行
g++ code.cpp
./a.out
 */

#include <iostream>
#include <vector>
#include <string>
#include <queue>
#include <deque>
#include <algorithm>
#include <utility>
#include <set>
#include <map>
#include <cmath>
#include <math.h>
#include <tuple>
#include <iomanip>

using namespace std;
typedef long long ll;
typedef long double ld;

int dy4[4] = {-1, 0, +1, 0};
int dx4[4] = {0, +1, 0, -1};
int dy8[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
int dx8[8] = {0, 1, 1, 1, 0, -1, -1, -1};

const long long INF = 1LL << 60;
const ll MOD = 1e9 + 7;

bool greaterSecond(const pair<int, int>& f, const pair<int, int>& s){
    return f.second > s.second;
}

ll gcd(ll a, ll b){
	if (b == 0)return a;
	return gcd(b, a % b);
}

ll lcm(ll a, ll b){
    return a / gcd(a, b) * b;
}

ll nCr(ll n, ll r){
    if(r == 0 || r == n){
        return 1;
    } else if(r == 1){
        return n;
    }
    return (nCr(n - 1, r) + nCr(n - 1, r - 1));
}

ll nPr(ll n, ll r){
    r = n - r;
    ll ret = 1;
    for (ll i = n; i >= r + 1; i--) ret *= i;
    return ret;
}

//-----------------------ここから-----------

int main(void){
    int k, n, f;
    cin >> k >> n >> f;

    int ans = k * n;

    vector<int> a(f);

    for(int i = 0; i < f; i++) cin >> a[i];

    for(int i = 0; i < f; i++){
        ans -= a[i];
        if(ans < 0){
            cout << -1 << endl;
            return 0;
        }
    }

    cout << ans << endl;


}