#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int,int>;
#define rep2(i,a,n) for(int i = (a);i < (n);i++)
#define rep(i,n) rep2(i,0,n)
#define INF (1LL << 60)

int main(void){
    int k,n,f;cin >> k >> n >> f;
    int a = 0;
    rep(i,f){
        int b;
        cin >> b;
        a += b;
    }
    
    k *= n;
    cout << max(-1,k-a) << endl;
    
}