#include <iostream>
#include <algorithm>
using namespace std;
const int N = 1010;
int a[N];
int main(){
    int n;  scanf("%d", &n);
    for(int i = 0; i < n; i ++ )    scanf("%d", &a[i]);
    sort(a, a + n);
    int cnt = 0;
    for(int i = 1; i < n; i ++ ){
        if(a[i] - a[i - 1] == 1)    cnt ++ ;
    }
    cout << cnt << endl;
    return 0;
}