#include<iostream>
using namespace std;
int main(){
    int K,N,F;
    int ans;
    cin>>K>>N>>F;
    ans=K*N;
    for(int i=0;i<F;i++){
        int x;
	cin>>x;
	ans-=x;
    }
    if(ans>=0)cout<<ans<<endl;
    else cout<<-1<<endl;
    return 0;
}