#include<bits/stdc++.h>
using namespace std;
using ll = long long;

int main(){
    int n;
    cin>>n;
    vector<int> a(n);
    for(int i = 0;i<n;i++) cin>>a[i];
    ll ans = 0;
    ll ri = a[n-1];
    int ni = n-2;
    while(true){
        if(ni<0) break;
        if(a[ni]==ri-1){
            ri--;
            ni--;
            continue;
        }
        ans += ri - a[ni] - 1;
        ri = a[ni];
        ni--;
    }
    ans += ri - 1;
    cout<<ans<<endl;
}