#include <iostream>
#include <cmath>
#include <vector>
#include <algorithm>
#define REP(i, n) for(int i = 0; i < (int)(n); i++)
#define FOR(i, j, k) for(int i = (int)(j); i < (int)(k); ++i)
#define ROF(i, j, k) for(int i = (int)(j); i >= (int)(k); --i)
#define SORT(v, n) sort(v, v+n)
#define REVERSE(v) reverse((v).begin(), (v).end())
#define print(x) cout << x << endl;
using namespace std;
const int MOD = (int)1e9 + 7;
int main(void){
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int K, N, F;
    cin >> K >> N >> F;
    vector<int> A(F);
    int T=0;
    REP(i, F){
        cin >> A[i];
        T += A[i];
    }
    
    if(T>K*N){
        print(-1);
    }else{
        print(K*N-T);
    }
    return 0;
}