/*
User ID: Tanmoy_Datta
Link : http://codeforces.com/problemset/problem/508/C
*/

#include <bits/stdc++.h>

#define pii pair <int,int>
#define sc scanf
#define pf printf
#define Pi 2*acos(0.0)
#define ms(a,b) memset(a, b, sizeof(a))
#define pb(a) push_back(a)
#define MP make_pair
#define oo 1<<29
#define dd double
#define ll long long
#define EPS 10E-10
#define ff first
#define ss second
#define MAX 120
#define SZ(a) (int)a.size()
#define getint(a) scanf("%d",&a)
#define loop(i,n) for(int i=0;i<n;i++)
#define all(a) a.begin(),a.end()
#define intlim 2147483648
#define inf 100000
#define rtintlim 46340
#define llim 9223372036854775808
#define rtllim 3037000499
#define ull unsigned long long
#define I int

using namespace std;
ll ara[350];
int main()
{
    ///freopen("in.txt","r",stdin);
    ///freopen("out.txt","w",stdout);
    ms(ara,-1000000);
    int m,t,r;
    cin>>m>>t>>r;
    vector<int>v;
    int num;
    for(int i=0; i<m; i++)
    {
        cin>>num;
        v.pb(num);
    }
    if(t<r)
        cout<<-1<<endl;
    else
    {
        int ans=0;
        //reverse(all(v));
        for(int i=0; i<m; i++)
        {
            int c=0;
            for(int j=1; j<=r; j++)
            {
                if(ara[j]+t>=v[i])
                    c++;
            }
            if(c==r)
                continue;
            else
            {
                //ans+=c;
                c=0;
                for(int j=1; j<=r; j++)
                {
                    if(ara[j]+t<v[i])
                    {
                        ara[j]=v[i]-(++c);
                        ans++;
                    }
                }

            }
        }
        cout<<ans<<endl;
    }
    return 0;
}