import Foundation

private func readString() -> String {
    readLine()!
}

private func readStringArray() -> [String] {
    readString().components(separatedBy: " ")
}

private func readInt() -> Int {
    Int(readString())!
}

private func readIntArray() -> [Int] {
    readStringArray().map { Int($0)! }
}

private func readStringTupple() -> (String, String) {
    let array = readStringArray()
    return (array[0], array[1])
}

private func readStringTupple() -> (String, String, String) {
    let array = readStringArray()
    return (array[0], array[1], array[2])
}

private func readIntTupple() -> (Int, Int) {
    let array = readIntArray()
    return (array[0], array[1])
}

private func readIntTupple() -> (Int, Int, Int) {
    let array = readIntArray()
    return (array[0], array[1], array[2])
}

private func pow(_ x: Int, _ y: Int) -> Int {
    Int(pow(Double(x), Double(y)))
}

/*＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿
 〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓
 ￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣*/

let n = readInt()
let a = [-1] + readIntArray()
var ans = 0
for i in 4 ..< n + 1 {
    for j in 1 ..< i {
        if i - a[i] == j + a[j] {
            ans += 1
        }
    }
}
print(ans)

/*＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿
〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓〓
￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣*/
