#include<bits/stdc++.h>
using namespace std;
const int MXN = 2e5+5;
int n, a[MXN];
long long dp[MXN];
int32_t main() {
cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
cin >> n;
for(int i=1; i<=n; i++) {
cin >> a[i];
dp[i] = dp[i-1] + (a[i-1]>=a[i] ? a[i-1]-a[i]+1 : 0);
}
long long ans = 1e18, val=0;
for(int i=n; i>=1; i--) {
if(a[i]+dp[i]<=a[i+1])
ans = min(ans, dp[i]+val + a[i+1]-a[i]-dp[i] + 1);
else
ans = min(ans, dp[i]+val - min({dp[i], (i<n && a[i+2]>=a[i+1] ? a[i+2]-a[i+1]+1 : 0ll), a[i]+dp[i]-a[i+1]-1}));
val += (a[i+1]>=a[i] ? a[i+1]-a[i]+1 : 0);
}
cout << ans << '\n';
return 0;
}
