#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;

const LL MOD=1000000007;

int main(){
    LL K, N, F;
    cin >> K >> N >> F;
    LL ans = K*N;
    for(int i = 0; i < F; i++){
        LL A;
        cin >> A;
        ans -= A;
    }
    if(ans < 0) ans = -1;
    cout << ans << endl;
    return 0;
}