#include "bits/stdc++.h"
using namespace std;
#define repi(i,a,b) for(int i=(a), i##_len=(b); i<i##_len; ++i)
#define rep(i,n) repi(i,0,n)
#define repi_(i,a,b) for(int i=(a), i##_len=(b); i<=i##_len; ++i)
#define rep_(i,n) repi_(i,0,n)
#define all(x) (x).begin(),(x).end()
#define vsort(v) sort((v).begin(), (v).end())
#define gsort(v) tsort(v); reverse((v).begin(), (v).end())
#define vrev(v) reverse((v).begin(), (v).end())
#define siz(v) ((int)(v).size())
#define ers(v, n) (v).erase((v).begin() + n)
#define cnt(v, n) count(all(v), n)
#define vmin(v) *min_element(v.begin(), v.end())
#define vmax(v) *max_element(v.begin(), v.end())
#define el "\n"
#define co(x) cout<<(x)<<el
#define coc(c, a, b) if(c)co((a));else co((b))
#define cosp(x) cout<<(x)<<' '
#define YES(c) coc(c,"YES", "NO")
#define Yes(c) coc(c,"Yes", "No")
#define yes(c) coc(c,"yes", "no")
#define POSSIBLE(c) coc(c, "POSSIBLE", "IMPOSSIBLE")
#define Possible(c) coc(c, "Possible", "Impossible")
#define possible(c) coc(c, "possible", "impossible")
#define inf INT_MAX
#ifdef tqktmp_2
#define gc() getchar(); getchar()
#else
#define gc() 1
#endif


typedef long long lint;

lint dx[8] = { 0,1,1,1,0,-1,-1,-1 },
dy[8] = { 1,1,0,-1,-1,-1,0,1 };

template<class T>inline bool maxi(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>inline bool mini(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }



int main() {
	cin.tie(0);
	ios::sync_with_stdio(0);

	lint k, n, f; cin >> k >> n >> f;
	lint all = k * n;
	lint sum = 0, in;
	rep(i, f) {
		cin >> in;
		sum += in;
	}
	if (all < sum)co(-1);
	else co(all - sum);
	gc();
}