#include<bits/stdc++.h>
#define rep(i,a,n) for (int i = a;i < n;i++)
#define per(i,n,a) for (int i = n-1;i >= a;i--)
using namespace std;

int main() {
    int k,n,f,sum = 0; cin >> k >> n >>f;
    vector<int> a(f);
    rep(i,0,f){
        cin >> a[i];
        sum += a[i];
    }
    if(k*n - sum >= 0){
        cout << k*n - sum << endl;
    }else{
        cout << -1 << endl;
    }

}