#include<bits/stdc++.h>
using namespace std;
int L, n, m;
int a[50005];
int search(int m){
int l = 0, r = L, mid, ans;
while(l <= r){
mid = (l + r)/2;
int now = 0, cnt = 0;
for(int i = 1; i <= n; i++){
if(a[i] - a[now] < mid){
cnt++;
}else{
now = i;
}
}
if(cnt <= m) ans = mid, l = mid + 1;
else r = mid - 1;
}
return ans;
}
int main(){
cin >> L >> n >> m;
for(int i = 1; i <= n; i++){
cin >> a[i];
}
cout << search(m);
return 0;
}
