#include <bits/stdc++.h>
using namespace std;
#define int long long
int leftside[200001], rightside[200001];
int32_t main() {
int n; cin >> n;
vector<int> arr(n);
for (int i=0;i<n;i++) {
cin >> arr[i];
}
leftside[0] = 0;
for (int i=1;i<n;i++) {
leftside[i] = leftside[i-1] + max(arr[i-1] - arr[i] + 1, (int)0);
}
rightside[n-1] = 0;
for (int i=n-2;i>-1;i--) {
rightside[i] = rightside[i+1] + max(arr[i+1] - arr[i] + 1, (int)0);
}
int ans = INT_MAX, curans;
if (leftside[n-1] > rightside[0]) {
ans = rightside[0];
} else {
ans = leftside[n-1];
}
for (int i=1;i<n-1;i++) {
curans = leftside[i-1] + rightside[i+1];
if (arr[i+1] - arr[i] - leftside[i-1] > arr[i-1] - arr[i] - rightside[i+1]) {
curans += max(arr[i+1] - arr[i] - leftside[i-1] + 1, (int)0);
} else {
curans += max(arr[i-1] - arr[i] - rightside[i+1] + 1, (int)0);
}
if (curans < ans) {
ans = curans;
}
}
cout << ans;
}
