    #include <stdio.h>
    #define N 200001
    int r[2 * N] = {0};
     
    int main() {
        int n, a[N], t;
        long long int u, s = 0;
        scanf("%d", &n);
        for (int i = 1; i <= n; i++) {
            scanf("%lld", &u);
            if (u < N) {
                t = i - u;
                if (0 <= t && t < 2 * N) r[t]++;
                a[i] = (int) u;
            } else a[i] = 0;
        }
        for (int i = 1; i < n; i++) {
            u = a[i] + i;
            if (a[i] && u < 2 * N) s += r[u];
        }
        printf("%d", s);
        return 0;
    }