#include<bits/stdc++.h>
using namespace std;
#define IOS cin.tie(nullptr)->sync_with_stdio(0),cin.exceptions(cin.failbit);
#define lb(x) (x)&-(x)
#define all(x) (x).begin(),(x).end()
#define ll long long
constexpr int maxN=2e5+5;
int n;
ll val[maxN],l[maxN],r[maxN];
int main(){
IOS
cin>>n;
for(int i = 1;i<=n;i++)cin>>val[i];
for(int i = 1;i<=n;i++){
l[i] = l[i-1];
if(val[i-1]>=val[i])l[i]+=val[i-1]-val[i]+1;
}
for(int i = n;i>=1;i--){
r[i] = r[i+1];
if(val[i+1]>=val[i])r[i]+=val[i+1]-val[i]+1;
}
ll ans = 1e18;
for(int i = 1;i<=n;i++)ans = min(ans,max(l[i],r[i]));
cout<<ans<<'\n';
}
