#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef __int128 lll;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<ld, ld> pld;
#define MAX 9223372036854775807LL
#define MIN -9223372036854775807LL
#define INF 0x3f3f3f3f3f3f3f3f
#define fi first
#define se second
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); cout << fixed; cout.precision(10);
#define sp << " "
#define en << "\n"
#define compress(v) sort(v.begin(), v.end()), v.erase(unique(v.begin(), v.end()), v.end())
ll n;
ll a[200010];
ll b[200010];
ll ans = INF;
ll sum1, sum2;
int main(void)
{
fastio
cin >> n;
for(ll i = 1 ; i <= n ; i++)
cin >> a[i];
for(ll i = 2 ; i <= n ; i++)
b[i - 1] = a[i - 1] - a[i];
for(ll i = 1 ; i < n ; i++)
{
if(b[i] <= 0)
sum2 += -b[i] + 1;
}
for(ll i = 0 ; i < n ; i++)
{
ans = min(ans, max(sum1, sum2));
if(b[i + 1] >= 0)
sum1 += b[i + 1] + 1;
if(b[i + 1] <= 0)
sum2 -= -b[i + 1] + 1;
}
cout << ans;
return 0;
}
