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

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int k,n,f;
    cin >> k >> n >> f;
    
    int sum_age = 0;
    rep(i,f) {
        int a;
        cin >> a;
        sum_age += a;
    }
    
    if(k * n < sum_age) cout << -1 << endl;
    else cout << k * n - sum_age << endl;
    
}