#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #ifdef _MSC_VER #include #endif #define FOR(i, a, b) for(int i = (a); i < (int)(b); ++i) #define rep(i, n) FOR(i, 0, n) #define ALL(v) v.begin(), v.end() #define REV(v) v.rbegin(), v.rend() #define MEMSET(v, s) memset(v, s, sizeof(v)) #define UNIQUE(v) (v).erase(unique(ALL(v)), (v).end()) #define MP make_pair #define MT make_tuple using namespace std; typedef long long ll; typedef long double ld; typedef pair P; int main(){ cin.tie(0); ios::sync_with_stdio(false); cout.setf(ios::fixed); cout.precision(20); ll k, n, f; cin >> k >> n >> f; vector a(f); rep(i, f) cin >> a[i]; ll sum = accumulate(ALL(a), 0ll); if (k*n < sum) cout << -1 << endl; else cout << k*n%sum << endl; }