#include<bits/stdc++.h>
#define rep(i,l,n) for (int i = (l); i < (n); i++)
using ll = long long;
using namespace std;
int main(){
    int k, n, f;
    cin >> k >> n >> f;
    int num = 0;
    rep(i,0,f) {
        int a;
        cin >> a; 
        num += a;
    }
    cout << max(-1,k * n - num) << endl;
}