#include<bits/stdc++.h>
using namespace std;
int main(){
int n;
long long ans, mn = (long long)1e9;
cin >> n;
long long a[n];
vector<long long> pos(n);
vector<long long> neg(n);
for(int i = 0; i < n; i++) cin >> a[i];
for(int i = 1; i < n; i++){
if(a[i] > a[i-1]) pos[i] = pos[i-1];
else if(a[i] < a[i-1]) pos[i] = pos[i-1] + a[i-1] - a[i] + 1;
else pos[i] = pos[i-1] + 1;
}
for(int i = n-2; i >= 0; i--){
if(a[i+1] < a[i]) neg[i] = neg[i+1];
else if(a[i+1] > a[i]) neg[i] = neg[i+1] + a[i+1] - a[i] + 1;
else neg[i] = neg[i+1] + 1;
}
for(int k = 0; k < n; k++){
if(neg[k] < pos[k]) ans = pos[k];
else ans = pos[k] + neg[k];
mn = min(mn,ans);
}
cout << mn << endl;
return 0;
}
