#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

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