//https://www.instagram.com/_modwwe/
#pragma GCC optimize("Ofast,unroll-loops")
//#pragma GCC target("avx2,bmi,bmi2")
#include<bits/stdc++.h>
#define int long long
//#define ll long long
#define down cout<<'\n';
#define NHP ios_base::sync_with_stdio(0);cout.tie(0);cin.tie(0);
#define modwwe int t;cin>>t; while(t--)
#define bit(i,j) (i>>j&1)
#define sobit(a) __builtin_popcountll(a)
#define task "test"
#define fin(x) freopen(x".inp","r",stdin)
#define fou(x) freopen(x".out","w",stdout)
#define pb push_back
#define checktime cerr << (double)clock() / CLOCKS_PER_SEC * 1000 << " ms";
using namespace std;
void phongbeo();
const int inf=1e18;
const int mod2=1e9+7;
const int mod1=998244353;
struct icd
{
int a,b;
};
struct ib
{
int a;
int b;
};
struct ic
{
int a,b,c;
};
struct id
{
int a,b,c,d;
};
struct ie
{
int a,b,c, d,e,f;
};
int n,m,s1,s2,s4,s3,sf,k,r,mid,s5,s6,mx,s7,s8,s9,mx2,res,dem2=0,dem=0,l;
int i,s10,s12;
int el=29;
main()
{
#ifndef ONLINE_JUDGE
/// fin(task),fou(task);
#endif
NHP;
/// cin>>s1;
// modwwe
phongbeo(),down
}
int a[200002];
int b[200002];
int c[200002];
void phongbeo()
{
cin>>n;
for(int i=1;i<=n;i++)
cin>>a[i];
s2=a[1];
for(int i=1;i<=n;i++){
b[i]=b[i-1]+max(a[i-1]-a[i]+1ll,0ll);
}
for(int i=n;i>=1;--i)
{ c[i]=c[i+1]+max(0ll,a[i+1]-a[i]+1ll);
}
s3=inf;
for(int i=1;i<=n;i++){
s3=min({s3,max(b[i],c[i])});
}
cout<<s3;
}
