#include <bits/stdc++.h>
using namespace std;
#define all(x) x.begin(),x.end()
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
template<class T> inline bool chmax(T&a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T> inline bool chmin(T&a,T b){if(a>b){a=b;return 1;}return 0;}
using ll = long long;

int k,n,f,a,sum;

int main(){
    cin >> k >> n >> f;
    rep(i,f) cin >> a, sum+=a;
    cout << (k*n>=sum ? k*n-sum : -1) << endl;
}