#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define fi first
#define se second
#define p(x,y) pair<ll,ll>(x,y)
#define BIT(i,x) ((x>>i)&1)
#define MASK(x) (1<<x)
#define ld long double
#define __builtin_popcount __builtin_popcountll
#define pll pair<ll,ll>
template<class T1,class T2>
bool maximize(T1 &x,const T2 &y)
{
if(x<y)
{
x=y;
return 1;
}
return 0;
}
template<class T1,class T2>
bool minimize(T1 &x,const T2 &y)
{
if(x>y)
{
x=y;
return 1;
}
return 0;
}
void online()
{
std::ios_base::sync_with_stdio(0);
cin.tie(0);
#ifdef thuc
freopen("input.INP","r",stdin);
freopen("output.OUT","w",stdout);
#endif
}
const ll N=2e5+10,M=998244353;
ll a[N],n,l[N],r[N],f[N];
int main()
{
online();
cin>>n;
ll ans=1e18;
for(int i=1;i<=n;i++)
{
cin>>a[i];
f[i]=f[i-1]+max(0ll,a[i-1]+1-a[i]);
}
for(int i=n;i>=1;i--)
{
minimize(ans,max(f[i],f[i+1]+max(0ll,a[i+1]+1-a[i])));
f[i]=f[i+1]+max(0ll,a[i+1]+1-a[i]);
}
cout<<ans;
}
