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