#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int a[f];
    int sum = 0;
    rep(i,f){
        cin >> a[i];
        sum += a[i];
    }

    int ans = k * n - sum;

    if(ans < 0)ans = -1;


    cout << ans << endl;
    return 0;
}