#include <iostream>
#define P(x) cout <<x<<endl
using namespace std;
int main(){
    int k,n,f,t;
    std::cin >> k >> n >>f;
    int mame = k*n;
    for(int i=0;i<f;i++){
        cin >> t;
        mame =mame-t;
    }
    if(mame<0) mame =-1;
    P(mame);
}