#include<bits/stdc++.h>
using namespace std;
int main(){
    int K,N,F;
    cin >> K >> N >> F;
    vector<int> family(F);
    for(int i=0;i<F;i++){
        cin >> family[i];
    }
    int mame = 0;
    for(int i=0;i<F;i++){
        mame += family[i];
    }
    if(mame <= K * N){
        cout << K * N - mame << endl;
    }
    else{
        cout << -1 << endl;
    }
}