#include "bits/stdc++.h" using namespace std; using ll = long long; const double pi = acos(-1); #define FOR(i,a,b) for (ll i=(a),__last_##i=(b);i<__last_##i;i++) #define RFOR(i,a,b) for (ll i=(b)-1,__last_##i=(a);i>=__last_##i;i--) #define REP(i,n) FOR(i,0,n) #define RREP(i,n) RFOR(i,0,n) #define __GET_MACRO3(_1, _2, _3, NAME, ...) NAME #define rep(...) __GET_MACRO3(__VA_ARGS__, FOR, REP)(__VA_ARGS__) #define rrep(...) __GET_MACRO3(__VA_ARGS__, RFOR, RREP)(__VA_ARGS__) template ostream& operator<<(ostream& os, const vector& v) { REP(i, v.size()) { if (i)os << " "; os << v[i]; }return os; } template ostream& operator<<(ostream& os, const vector>& v) { REP(i, v.size()) { if (i)os << endl; os << v[i]; }return os; } int main() { ll k, n, f, a; ll sum = 0; cin >> k >> n >> f; REP(i, f) { cin >> a; sum += a; } if (k * n < sum) { cout << -1 << endl; } else { cout << k * n - sum << endl; } }