#include<bits/stdc++.h> #define ll long long using namespace std; int main() { int n; cin>>n; vector<ll>v(n); for(int i=0; i<n; i++) cin>>v[i]; if(n==1) { cout<<0; return 0; } vector<ll>p(n,0); for(int i=1; i<n; i++) { ll d=v[i]-v[i-1]; p[i]=p[i-1]+max(0LL,1-d); } vector<ll>s(n,0); for(int i=n-2; i>=0; i--) { ll d=v[i]-v[i+1]; s[i]=s[i+1]+max(0LL,1-d); } ll mini=-1; for(int i=0; i<n; i++) { ll maks=max(p[i],s[i]); if(mini==-1 || maks<mini) mini=maks; } cout<<mini; }