#include <iostream>
using namespace std;
int main () {
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
int a[n];
for (int i=0; i<n; i++) {
cin >> a[i];
}
long long pre[n];
pre[0] = 0;
for (int i=1; i<n; i++) {
pre[i] = pre[i-1]+max(0, a[i-1]-a[i]+1);
}
long long suf[n];
suf[n-1] = 0;
for (int i=n-2; i>=0; i--) {
suf[i] = suf[i+1]+max(0, a[i+1]-a[i]+1);
}
long long o = 1e16;
for (int i=0; i<n; i++) {
long long cost = 0;
if (i > 0) cost += pre[i-1];
if (i < n-1) cost += suf[i+1];
if (i > 0 && i < n-1) cost += max(0ll, max(a[i-1]-(a[i]+suf[i+1])+1, a[i+1]-(a[i]+pre[i-1])+1));
o = min(o, cost);
}
cout << o << endl;
}
