#include <bits/stdc++.h> using namespace std; #define int long long signed main() { int n; cin>>n; vector<int>a(n); for(int i = 0;i<n;i++){ cin>>a[i]; } vector<int>pref(n+1,0); for(int i = 2;i<=n;i++){ if(a[i-1]<=a[i-2]+1){ pref[i] = pref[i-1]+a[i-2]+1-a[i-1]; } else{ pref[i] = pref[i-1]; } } vector<int>suff(n+1,0); for(int i = n;i>=2;i--){ if(a[i-1]<=a[i]+1){ suff[i] = suff[i+1]+a[i]+1-a[i-1]; } else{ suff[i] = suff[i+1]; } } int ans = 0; for(int i = 1;i<=n;i++){ ans = min(ans,max(pref[i],suff[i])); } cout<<ans; }