#include<iostream>
using namespace std;

int N, K, F;
int A;
int main() {
int i;
cin >> N >> K >> F;
int diff = N * K;    //袋とか考えずに豆の数だけに注目したい
for( i = 0; i < F; i++ ) {
cin >> A;
diff -= A;
}
if (diff < 0)
cout << -1;
else
cout << diff;
cout << endl;
return 0;
}