#include <bits/stdc++.h>
using namespace std;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
long long n;cin >> n;
vector<long long> A(n);
for(int i = 0;i<n;i++){
cin >> A[i];
}
//sequence has to be increasing then decreasing
vector<long long> p(n);
vector<long long> s(n);
p[0] = A[0];
//I can water a consecutive segment
for(int i = 1;i<n;i++){//has to be increasing
p[i] = max(A[i],p[i-1] + 1);
}
s[n-1] = A[n-1];
for(int i = n-2;i>=0;i--){
s[i] = max(A[i],s[i+1] + 1);
}
vector<long long> B(n);
B[0] = 0;
for(int i = 1;i<n;i++){
B[i] = B[i-1] + max(0LL, p[i] - A[i]);
}
//B[i] represents the costs
vector<long long> Z(n);
Z[n-1] = 0;
for(int i = n-2;i>=0;i--){
Z[i] = Z[i+1] + max(0LL,s[i] - A[i]);
}
long long ans = LLONG_MAX;
for(int i = 0;i<n;i++){
ans = min(ans,max(Z[i],B[i]));
}
cout << ans;
}
