#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <set>
#include <sstream>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cctype>
#include <climits>
#include <iostream>
#include <iomanip>
#include <functional>
using namespace std;
typedef signed long long ll;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(int x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
//-------------------------------------------------------

int main()
{
    int k, n, f;
    cin >> k >> n >> f;
    vector<int> as(f);
    int sum = 0;
    for(int i = 0; i < f;i++)
    {
        cin >> as[i];
        sum += as[i];
    }
    if(k * n >= sum)
    {
        cout << k * n - sum << endl;
        return 0;
    }
    cout << -1 << endl;
    return 0;
}