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

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