#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimize("Ofast,unroll-loops")
using namespace std;
using namespace __gnu_pbds;
#define pb push_back
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define f first
#define s second
#define int long long
#define ll long long
#define pii pair<int,int>
#define ar array
template<class T>bool umax(T &a,T b){if(a<b){a=b;return true;}return false;}
template<class T>bool umin(T &a,T b){if(b<a){a=b;return true;}return false;}
template<class T> using st = tree<T, null_type, less_equal<T>,
rb_tree_tag, tree_order_statistics_node_update>;
const int inf = 1e17 + 7;
const int mod = 1e9 + 7;
const int N = 3e5 + 5;
const int md = 998244353;
int binpow(int a, int b){
if(b == 0)return 1;
if(b % 2 == 0){
int c = binpow(a,b/2);
return (c*c)%mod;
}
return (binpow(a,b-1)*a)%mod;
}
int divi(int a, int b){
return (a*(binpow(b,mod-2)))%mod;
}
int n,m,k;
void solve(){
cin >> n;
vector<int>v(n);
for(auto &to:v)cin >> to;
vector<int>pref(n+2),suff(n+2);
for(int i = 2;i<=n;i++){
pref[i] = pref[i-1];
pref[i] += max(0ll, v[i-2]-v[i-1] + 1);
}
for(int i = n-1;i>=1;i--){
suff[i] = suff[i+1];
suff[i] += max(0ll, v[i]-v[i-1]+1);
}
int ans = inf;
for(int i = 1;i<=n;i++){
//~ cout << pref[i] << " ";
umin(ans, max(pref[i] , suff[i]) );
}
//~ cout << "\n";
//~ for(int i = 1;i<=n;i++){
//~ cout << suff[i] << " ";
//~ }
cout << ans << "\n";
}
/*
*/
signed main()
{
// freopen("seq.in", "r", stdin);
// freopen("seq.out", "w", stdout);
ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
int tt=1;//cin>>tt;
while(tt--)solve();
}
