#include <iostream>
#include <fstream>
#define INF 100008
#define NMAX 100000
using namespace std;



ifstream f("numarare.in");
ofstream g("numarare.out");

int a[ 2000001 ], i, n, s;
long long p[ 2000001 ];

int main()
{
    f >> n;
    for( i = 1; i <= n; ++i )f >> a[ i ];

    for( i = 1; i <= n; ++i ){
        p[ i ] = 0;
        while( a[ i + p [ i ] + 1 ] + a[ i - p[ i ] ] == a[ i + 1 ] + a[ i ] )p [ i ]++;
    }

    for( i = 1; i <= n; ++i ) {
            s = s + p[ i ];
    }
    g << s - 1;

    return 0;
}