#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
#define endl "\n"
#define creeper_bonc ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define pb push_back
#define fi first
#define se second
#define pii pair<int, int>
#define all(v) (v).begin(), (v).end()
#define ins insert
#define nl endl
#define int long long
const int imax = LLONG_MAX;
const int imin = LLONG_MIN;
void _()
{
int n; cin>>n;
vector<int> arr(n + 1), pre(n + 2, 0), suf(n + 2, 0);
for(int i = 1;i <= n;i++) cin>>arr[i];
for(int i = 2;i <= n;i++)
{
pre[i] = pre[i - 1];
if(arr[i] <= arr[i - 1]) pre[i] += (arr[i - 1] - arr[i] + 1);
}
for(int i = n - 1;i >= 1;i--)
{
suf[i] = suf[i + 1];
if(arr[i] <= arr[i + 1]) {
suf[i] += (arr[i + 1] - arr[i] + 1);
}
}
int ans = imax;
for(int i = 1;i <= n;i++) ans = min(ans, max(pre[i], suf[i]));
cout<<ans;
}
signed main()
{
creeper_bonc
int t = 1;
//cin >> t;
while(t--) {
_();
cout << endl;
}
return 0;
}
/*Author: Davud*/
