#include <cstdio> 
#include <iostream>
#include <algorithm>
using namespace std;
const int INF= 0x3f3f3f3f ;
int d[550000]; 
int L;
int calc(int x)
{
    int now=0; 
    int cnt=0;
    int ps=0;
    
    while(now< L)
    {
        now+= x;
        while(now >=d[ps+1]) ps++;
        now=d[ps];
        cnt++;
    } 
    return cnt;
} 
int main()
{
    int n, m;
    while(scanf("%d%d%d", &L, &n, &m) != EOF)
    {
        int minx =0 ;
        for(int i=1; i<= n; i++) scanf("%d", &d[i]);
        sort(d+1, d+1+n);
        d[0]= 0; d[n+1]= L; 
        d[n+2]= INF ; 
        for(int i=1; i<= n+1; i++)
            if(minx <d[i]- d[i-1])
                minx= d[i]- d[i-1];
        
        int left= minx, right= L;
        int res; 
        while(left <= right){
            int mid= (left+right)>> 1;
            if(calc(mid) <= m)
            {
                res= mid;
                right= mid-1;
            }
            else left= mid+1;
        } 
        printf("%d\n", res);
    }
    return 0;
}