#include<iostream>
#include<algorithm>
#include<stdio.h>
#include<string.h>
#include<stdlib.h>

using namespace std;

struct node
{
    int x;
    int y;
}q[3010];

int a[3001];
int n,m,k;

int main()
{
    while(scanf("%d%d%d",&n,&m,&k)!=EOF)
    {
            memset(a,0,sizeof(a));
            memset(q,0,sizeof(q));
            for(int i=0;i<n;i++)
            {
                scanf("%d",&a[i]);
            }
            if(m<k)
            {
                printf("-1\n");
                continue;
            }
            int kk = m;
            int sum = 0;
            int flag = 0;
            for(int i=0;i<n;i++)
            {
                if(i == 0)
                {
                    sum = k;
                    for(int j=a[i]-k;j<a[i];j++)
                    {
                        q[j].y = 1;
                    }
                    for(int j=a[i];j<a[i]+m;j++)
                    {
                        q[j].x = kk;
                        kk--;
                    }
                }
                else
                {
                    if(q[a[i]].x < k)
                    {
                        //printf("q[%d].x = %d\n",a[i],q[a[i]].x);
                        int cnt = k - q[a[i]].x;
                        sum += cnt;
                        for(int j=a[i]-cnt;j<a[i];j++)
                        {
                            if(q[j].y == 1)
                            {
                                flag = 1;
                                break;
                            }
                            q[j].y = 1;
                            for(int v=j+1;v<m+j+1;v++)
                            {
                                q[v].x++;
                            }
                        }
                    }
                }
                if(flag == 1)
                {
                    break;
                }
            }
            if(flag == 1)
            {
                printf("-1\n");
            }
            else
            {
                printf("%d\n",sum);
            }

    }
    return 0;
}