#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int n, a[1001001];
int main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n;
for(int i=1;i<n;i++) cin >> a[i];
int mx = 0, flag = 0, R = 0;
for(int i=1;i<n;i++) {
if(mx+2 < a[i]) return cout << 0, 0;
if(mx+2 == a[i]) flag++, R = i;
mx = max(mx, a[i]);
}
if(flag > 1) return cout << 0, 0;
if(flag) {
int L = 1;
while(a[L] < mx-2) L++;
return cout << R-L, 0;
}
ll ans = 0; mx = 0;
for(int i=1;i<=n;i++) {
ans += mx+1, mx = max(mx, a[i]);
}
cout << ans - (n-1);
}
