#include<bits/stdc++.h>
using namespace std;
#define LL long long
#define U unsigned
#define F first
#define S second
#define PB push_back
#define FR front
#define B back
#define P push
#define C const
int n, l[200009], poc = 0, kraj, sum = 0;
int main(){
ios_base::sync_with_stdio(false); cin.tie(0); cin >> n; kraj = n+1;
for(int i=1; i<=n; i++) cin >> l[i];
l[0] = 0; l[kraj] = 0;
while(poc <= kraj){
while(l[poc] < l[poc+1]) poc++;
while(l[kraj] < l[kraj-1]) kraj--;
if (poc >= kraj) break;
sum += max(l[poc]-l[poc+1]+1, l[kraj-1]-l[kraj]+1); poc++; kraj--;
}
cout << sum; return 0;
}
