using namespace std;
#include <bits/stdc++.h>
#define BEGIN ios_base::sync_with_stdio(0);cin.tie(0);
#define END return EXIT_SUCCESS;
#define FOR(I,A,B) for(int (I)=(A);(I)<(B);++(I))
#define ALL(C) (C).begin(),(C).end()
inline void solve()
{
	int K,N,F,S;cin>>K>>N>>F;S=K*N;
	int T,A=0;FOR(i,0,F){cin>>T;A+=T;}
	if(S>=A)cout<<(S-A)<<endl;
	else cout<<(-1)<<endl;
}
int main(int argc,char**argv)
{
	BEGIN
	solve();
	END
}