#include <bits/stdc++.h>

using namespace std;

const int MAXN = 200001;

int a[MAXN], tree[2*MAXN], n;
vector<int> extra[MAXN];

void upd(int u) {
    u += n;
    tree[u]++;
    for (u /= 2; u >= 1; u /= 2) {
        tree[u] = tree[2*u]+tree[2*u+1];
    }
}

int qry(int l, int r) {
    l += n; r += n;
    int res = 0;
    while (l <= r) {
        if (l % 2 == 1) res += tree[l++];
        if (r % 2 == 0) res += tree[r--];
        l /= 2; r /= 2;
    }
    return res;
}


int main()
{
    cin >> n;

    for (int i = 1; i <= n; i++) {
        cin >> a[i];
        int l = max(1, i-a[i]);
        extra[l].push_back(i);
    }

    long long ans = 0;
    for (int i = 1; i <= n; i++) {
        ans += qry(i, n);
        for (int j : extra[i]) {
            ans -= qry(j, n);
        }
        upd(min(n, i+a[i]));
    }
    cout << ans << "\n";
    return 0;
}