#include <bits/stdc++.h>
using namespace std;
#define int long long

vector<int>t;

void update(int v, int tl, int tr, int pos, int val)
{
    if(tl == tr) t[v] += val;
    else
    {
        int tm = (tl+tr)/2;
        if(pos <= tm) update(v*2, tl, tm, pos, val);
        else update(v*2+1, tm+1, tr, pos, val);
        t[v] = t[v*2] + t[v*2+1];
    }
}

int query(int v, int tl, int tr, int l, int r)
{
    if(l > r) return 0;
    if(tl == l && tr == r) return t[v];
    int tm = (tl+tr) / 2;
    return query(v*2, tl, tm, l, min(tm, r)) + query(v*2+1, tm+1, tr, max(tm+1, l), r);
}

signed main() {
    ios::sync_with_stdio(false);cin.tie(nullptr);
	int n; cin >> n;
    int ans = 0;
    t.resize(4*n);
    set<pair<int, int>>z;
    for(int i = 1; i <= n; i++)
    {
        int x; cin >> x;
        while(!z.empty() && z.begin()->first < i)
        {
            update(1, 1, n, z.begin()->second, -1);
            z.erase(z.begin());
        }
        ans += query(1, 1, n, max(1LL, i-x), i);
        update(1, 1, n, i, 1);
        z.insert({i+x, i});
    }
    cout << ans;
}