#include <bits/stdc++.h>

using ll = long long;

using namespace std;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    ll k,n,f,r;
    cin >> k >> n >> f;

    r=k*n;
    for(ll i=0;i<f;++i)
    {
        ll a;
        cin >> a;
        r-=a;
    }

    if (r<0)
        r=-1;
    
    cout << r << "\n";

    return 0;
}