#include <bits/stdc++.h>
typedef long long ll;
using namespace std;

const int INF = 1e9;
const int MOD = 1e9+7;
const ll LINF = 1e18;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) for(int i=0;i<(n);++i)
#define COUT(x) cout<<(x)<<"\n"

int main(){
    int k,n,f;
    cin >> k >> n >> f;
    int all=0;
    REP(i,f){
        int a;
        cin >> a;
        all += a;
    }
    if(all>k*n)COUT(-1);
    else COUT(k*n-all);
}