#include <cstdio>
#include <iostream>
#include <fstream>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <cstring>
#include <string>
#include <ctime>
#include <cassert>
#include <utility>

using namespace std;

#define MAXN 32005

int A[MAXN];
int N;

int main() {
//	freopen("date.in", "r", stdin);
//	freopen("date.out","w", stdout);
	
	scanf("%d", &N);
	for(int i = 0; i < N; i++)
        scanf("%d", &A[i]);
	
	int ans1 = 0;
	for(int i = 0; i < N; i++)
        if(A[i] != (i & 1))
            ans1++;
    int ans2 = 0;
	for(int i = 0; i < N; i++)
        if(A[i] != (1 - (i & 1)))
            ans2++;
	
	int ans = min(ans1, ans2);
	printf("%d\n", ans);
	
	return 0;
}