//{{{
#include <bits/stdc++.h>
using namespace std;
#define rep(...) repN((__VA_ARGS__,rep3,rep2,loop,~))(__VA_ARGS__)
#define loop(n) rep2(_loop_,n)
#define rep2(i,n) rep3(i,0,n)
#define rep3(i,begin,end) for(int i=(int)(begin),i##_end=(int)(end);i<i##_end;++i)
#define repN(a) repX a
#define repX(a0,a1,a2,x,...) x
//}}}

int main(void){
	int K, N, F;
	cin >> K >> N >> F;
	N *= K;
	rep(i, F){
		int a;
		cin >> a;
		N -= a;
	}
	printf("%d\n", N >= 0 ? N : -1);
	return 0;
}