#include <iostream>
#include <algorithm>

using namespace std;

const int N = 1010;

int a[N];

bool is_true(int x,int y){
    if (y-x == 1) return true;
    else return false;
}

int main(){
    int n;cin>>n;
    int cnt = 0;
    for (int i = 0; i< n;i++){
        cin>>a[i];
    }
    sort(a,a+n);
    for (int i = 0;i<n-1;i++ ){
        if (is_true(a[i],a[i+1])) cnt++;
    }
    cout<<cnt<<endl;
    return 0;
}