#include <iostream>

using namespace std;
int n,sol1,sol2,v[32004],a[32004];
int main()
{
    cin>>n;
    for(int i=1; i<=n; i++)
    {
        cin>>v[i];
        a[i]=v[i];
    }
    for(int i=1; i<n; i++)
        if(v[i]==v[i+1]){sol1++;v[i+1]=(v[i]+1)%2;}
     for(int i=n; i>1; i--)
        if(a[i]==a[i-1]){sol2++;a[i-1]=(a[i]+1)%2;}

    cout<<min(sol1,sol2)<<"\n";
    return 0;
}