#include<stdio.h>
#include<queue>
#include<string.h>
#include<algorithm>
#include<iostream>
#include<queue>
#include<set>
#include<vector>
using namespace std;
const int MAXN = 500010;
int L, n, m;
int ston[MAXN];
bool js(int x){
    int cnt = 0, last = ston[0];
    for(int i = 1; i <= n; i++){
        if(ston[i] - ston[i - 1] > x)
            return false;
        if(ston[i] - last > x){
            cnt++;
            last = ston[i - 1];
            if(cnt >= m)
                return false;
        }
    }
    return true;
}
int erfen(int l, int r){
    int mid, ans;
    while(l <= r){
        mid = (l + r) >> 1;
        if(js(mid)){
            ans = mid;
            r = mid - 1;
        }
        else
            l = mid + 1;
    }
    return ans;
}
int main(){
    while(~scanf("%d%d%d", &L, &n, &m)){
        for(int i = 1; i <= n; i++)
            scanf("%d", ston + i);
        ston[0] = 0;
        ston[n + 1] = L;
        n++;
        sort(ston, ston + n + 1);
        printf("%d\n", erfen(0, L));
    }
    return 0;
}