#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
vector<int> b(n - 1);
for (int i = 0; i < n - 1; i++) {
cin >> b[i];
}
vector<int> mx(n - 1);
vector<bool> good(n - 1);
for (int i = 0; i < n - 1; i++) {
mx[i] = (i == 0 ? b[i] : max(mx[i - 1], b[i]));
if (i == 0) {
good[i] = (b[i] == 1);
} else {
good[i] = (good[i - 1] & (b[i] <= mx[i - 1] + 1));
}
}
long long ans = 0;
{
vector<int> seq;
seq.push_back(1);
for (int i = 0; i < n - 1; i++) {
seq.push_back(b[i]);
}
bool ok = (seq[0] == 1);
int mx = 1;
for (int i = 1; i < n; i++) {
if (seq[i] > mx + 1) {
ok = false;
break;
}
mx = max(mx, seq[i]);
}
ans += ok;
}
set<int> st;
for (int i = n - 1; i >= 1; i--) {
if (good[i - 1] && (int) st.size() < 2) {
if ((int) st.size() == 1) {
int x = *st.begin();
if (b[i - 1] != x && x <= mx[i - 1] + 1) {
ans += 1;
}
} else {
int c = mx[i - 1] + 1;
if (b[i - 1] <= c) {
c -= 1;
}
ans += c;
}
}
if (i > 1 && b[i - 1] > mx[i - 2] + 1) {
st.insert(b[i - 1] - 1);
}
}
cout << ans << '\n';
return 0;
}
