#include <iostream>
#include <iomanip>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <queue>
#include <map>
#include <unordered_map>
#include <stack>
#include <bitset>
#include <string>
#include <cstring>
#include <iterator>
#include <random>
#define ll long long
#define ld long double
#define inf (ll)(2*1e18)
#define sort(a) sort(a.begin(), a.end())
#define reverse(a) reverse(a.begin(), a.end())
#define pb push_back
#define endl "\n"
using namespace std;
void solve(){
ll n, i, l, r, res=inf, k;
cin>>n;
vector<ll> a(n);
vector<ll> b(n);
vector<ll> c(n);
for(i=0;i<n;++i){
cin>>a[i];
}
for(i=1;i<n;++i){
b[i] = a[i-1] + 1 - a[i];
b[i] = max(0ll, b[i]);
b[i] += b[i-1];
}
for(i=n-2;i>=0;--i){
c[i] = a[i+1] + 1 - a[i];
c[i] = max(0ll, c[i]);
c[i] += c[i+1];
}
for(i=0;i<n;++i){
res = min(res, max(b[i], c[i]));
}
cout<<res<<endl;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
srand(time(nullptr));
ll t=1;
// cin>>t;
for(;t>0;--t){
solve();
}
return 0;
}
