# include <bits/stdc++.h> using namespace std; long long l,n,m,a[1000000001]; bool check(long long x){ long long cnt = 0,last = 0; for(long long i = 1;i <= n + 1;i++){ if (a[i] - last < x) cnt++; else last = a[i]; }return cnt <= m; }long long BinarySearch(){ long long lt = -1,rt = l + 1; while (lt + 1 < rt) { long long mid = (lt + rt) >> 1; if (check(mid)) lt = mid; else rt = mid; }return lt; }int main(){ cin >> l >> n >> m; a[0] = 0; for(long long i = 1;i <= n;i++) cin >> a[i]; a[n+1] = l; cout << BinarySearch(); return 0; }