import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        int  n = 6;
		int[] P = new int[]{0,6,7,2,100,10};
        
        
        long c = 0;
        HashMap<Integer, Integer> freq = new HashMap<>();
        
        
        if (n > 0) {
            freq.put(P[0], 1);
        }

        
        for (int j = 1; j < n; j++) {
            int rhs = P[j] - j;
            c = c + freq.getOrDefault(rhs, 0);
            
            int prevValue= P[j - 1] - j + 1;
            freq.put(prevValue, freq.getOrDefault(prevValue, 0) + 1);
        }
        
        System.out.println(c);
        
    }
}