#include <iostream>
using namespace std;
int main(){
    int a,b,c;cin>>a>>b>>c;
    int sm = 0;
    for(int i = 0; c > i; i++){
        int tmp;cin>>tmp;
        sm += tmp;
    }
    cout << (a*b-sm < 0 ? -1:a*b-sm) << endl;
}