#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define f first
#define s second
#define x first
#define y second
#define elif else if
#define endl '\n'
#define all(x) begin(x), end(x)
ll n,a[200069];
ll dpl[200069],dpr[200069];
ll minimum=LLONG_MAX;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin>>n;
for (int i = 0; i < n; i++) {
cin>>a[i];
}
for (int i = 1; i < n; i++) {
dpl[i]=dpl[i-1]+max(a[i-1]+1-a[i],(ll)0);
}
for (int i = n-2; i >= 0; i--) {
dpr[i]=dpr[i+1]+max(a[i+1]+1-a[i],(ll)0);
}
for (int i = 0; i < n; i++) {
minimum=min(minimum,max(dpl[i],dpr[i]));
}
cout<<minimum<<endl;
return 0;
}
