#include<bits/stdc++.h>
using namespace std;
int main(){
    int K,N,M;
    cin>>K>>N>>M;
    K*=N;
    while(M--){
        int a;
        cin>>a;
        K-=a;
    }
    if(K<0)cout<<-1<<endl;
    else cout<<K<<endl;
}