#include <bits/stdc++.h>
#if 1 && defined(LOCAL)
#include <LOCAL/LOCAL.hpp>
#else
#define debug(...)
#define line
#endif
using namespace std;
typedef long long ll;
typedef long double ld;
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
template<typename T> inline bool chmax(T &a, T b) { return ((a < b) ? (a = b, true) : (false)); }
template<typename T> inline bool chmin(T &a, T b) { return ((a > b) ? (a = b, true) : (false)); }

int main() {
    int K,N,F;
    cin>>K>>N>>F;
    int kosuu=K*N;
    int sum=0;
    for(int i=0;i<F;i++){
        int a;
        cin>>a;
        sum+=a;
    }
    cout<<(kosuu<sum? -1:kosuu-sum)<<endl;
}