#pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define pb push_back
#define ff first
#define ss second
#define MOD 1000000009
#define INF 1000000019
#define INFL 1000000000000000099LL
ll n,q,s,t,a,b,c,ans=INFL,k,m;
ll co[1000007];
ll dp[1000007],dp2[1000007];//koszt,
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);
cin>>n;
for(ll i=0;i<n;i++){
cin>>a;
co[i]=a;
}
for(ll i=1;i<n;i++){
dp[i]=dp[i-1]+max(0LL,co[i-1]-co[i]+1);
}
for(ll i=n-2;i>=0;i--)
dp2[i]=dp2[i+1]+max(0LL,co[i+1]-co[i]+1);
for(ll i=0;i<n;i++)ans=min(ans,max(dp[i],dp2[i]));
cout<<ans;
}
