import java.util.*;

public class Solution {

    static Scanner sc = new Scanner(System.in);

    public static int pairs(int n) {
    	return (n * (n-1))/2;
    }

    public static int completeAssignment(int N,int[] A){
    	Map<Integer,Integer> hm = new HashMap<Integer,Integer>();

    	for(int i=0;i<N;i++) {
    		A[i]+=i;
    		hm.put(A[i], hm.getOrDefault(A[i], 0) + 1);
    	}
    	int res =0;

    	for(int i : hm.values()) {
    		res+=pairs(i);
    	}
    	return res;
    }

    public static void main(String[] args) {
    	int N = sc.nextInt();
    	int[] A = new int[N];

    	for(int i=0;i<N;i++) {
    		A[i] = sc.nextInt();
    	}

    	System.out.println(completeAssignment(N,A));
    }
}