#include<bits/stdc++.h>
using namespace std;
void PlayGround() {
int n;
cin>>n;
int a[n];
for(int i=0; i<n; ++i) {
cin>>a[i];
}
if(n==2) {
cout<<"0\n";
return;
}
long long b[n-1];
for(int i=1; i<n; ++i) {
b[i-1] = a[i]-a[i-1];
}
long long ans = LLONG_MAX;
vector<long long>A(n-1), B(n-1);
for(int i=0; i<n-1; ++i) {
A[i] = max(0LL, 1-b[i]);
B[i] = max(0LL, b[i]+1);
if(i) A[i] += A[i-1];
}
for(int i=n-3; i>=0; --i) {
B[i] += B[i+1];
}
ans = min(B[0], A[n-2]);
for(int i=0; i<n-2; ++i) {
ans = min(ans, max(A[i],B[i+1]));
}
cout<<ans<<'\n';
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
PlayGround();
return 0;
}
