#include<bits/stdc++.h>
using namespace std;
#define INIT ios_base :: sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define mp make_pair
#define pb push_back
#define ft first
#define sc second
#define ll long long
#define pii pair<int, int>
#define count_bits __builtin_popcount
#define int ll
inline char get_ (){
const int oo=1000005;
static char buf[oo], *p1 = buf, *p2 = buf;
return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, oo, stdin), p1 == p2) ? EOF : *p1 ++;
}
int read_ () {
char c = get_();
int sum = 0;
while(!(c >= '0' && c <= '9')) c = get_();
while(c >= '0' && c <= '9') sum = sum * 10 + (c - '0'), c = get_();
return sum;
}
int n, a[200010];
int st[200010], dr[200010];
int32_t main(){
INIT
cin>>n;
for(int i=1; i<=n; i++){
cin>>a[i];
}
for(int i=2; i<=n; i++){
st[i]=st[i-1];
if(a[i]<(a[i-1]+1) ){
st[i]+=a[i-1]+1-a[i];
}
}
for(int i=(n-1); i>=1; i--){
dr[i]=dr[i+1];
if(a[i]<(a[i+1]+1) ){
dr[i]+=a[i+1]+1-a[i];
}
}
int res=1e18;
for(int i=1; i<=n; i++){
res=min(max(st[i-1], dr[i+1] )+max(0ll, max(a[i-1]+1, a[i+1]+1)-a[i] ) , res );
}
cout<<res;
return 0;
}
