#include <stdio.h>

 #include <algorithm>

 int d[];

 int s, n, m;

 bool judge(int mid)

 {

     int sum = , cnt = ;

     for(int i = ; i <= n+; i++)

     {

         sum += d[i] - d[i-];

         if(sum < mid)

             cnt++;

         else

             sum = ;

     }

     if(cnt > m)

         return ;

     else return ;

 }

 int main()

 {

     scanf("%d %d %d", &s, &n, &m);

     for(int i = ; i <= n; i++)

     {

         scanf("%d", &d[i]);

     }

     d[] = ;

     d[n+] = s;

     std::sort(d, d+n+);

     int low = , high = s+, mid;

     while(low < high)

     {

         mid = (high + low) / ;

         if(judge(mid))

             high = mid;

         else

             low = mid + ;

     }

     printf("%d\n", low-);

     return ;

 }