#include<bits/stdc++.h>
#define endl '\n'
using namespace std;
const long long MAXN = 2e5 + 10;
void speed()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
}
long long n;
long long a[MAXN];
void read()
{
cin >> n;
for (long long i = 1; i <= n; ++ i)
cin >> a[i];
}
long long dp0[MAXN], dp1[MAXN];
void solve()
{
for (long long i = 2; i <= n; ++ i)
{
dp0[i] = dp0[i-1];
if(a[i] <= a[i-1])
{
dp0[i] += a[i-1] + 1 - a[i];
}
}
for (long long i = n-1; i >= 1; -- i)
{
dp1[i] = dp1[i+1];
if(a[i] <= a[i+1])
{
dp1[i] += a[i+1] + 1 - a[i];
}
}
long long ans = 1e17+10;
for (long long i = 1; i <= n; ++ i)
{
ans = min(max(dp0[i], dp1[i]), ans);
}
cout << ans << endl;
}
int main()
{
speed();
read();
solve();
return 0;
}
