#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],A[i-1] + 1);
}
s[n-1] = A[n-1];
for(int i = n-2;i>=0;i--){
s[i] = max(A[i],A[i+1] + 1);
}
vector<long long> B(n);
for(int i = 0;i<n;i++){
B[i] = min(p[i],s[i]);
}
//B[i] represents the heights
long long ans = 0;
long long offset = 0;
for(int i = n-1;i>=0;i--){
long long diff = B[i] - A[i];
ans += max(0LL,diff-offset);
offset += diff;
}
cout << ans;
}
