1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
#include <cstdio>
#include <algorithm>

using namespace std;

int a[50001];
int b[50001];

const int CURRENT_CURRENT = 0;
const int CURRENT_LOWEST = 1;
const int CURRENT_HIGHEST = 2;
const int LOWEST_CURRENT = 3;
const int LOWEST_HIGHEST = 4;
const int HIGHEST_CURRENT = 5;
const int HIGHEST_LOWEST = 6;

const int INF = 1000000001;

int dp[7];
int dp2[7];

//#include <ctime>

int main()
{
//  time_t t = clock();
  
  int n;
  scanf("%d", &n);

  for (int i = 0; i < n; i++)
  {
    scanf("%d", a + i);
    b[i] = a[i];
  }

  dp2[CURRENT_CURRENT] = INF;
  if (a[0] != a[1]) dp2[CURRENT_CURRENT] = 0;
  
  dp2[CURRENT_LOWEST] = 1;
  dp2[CURRENT_HIGHEST] = 1;
  dp2[LOWEST_CURRENT] = 1;
  dp2[LOWEST_HIGHEST] = 2;
  dp2[HIGHEST_CURRENT] = 1;
  dp2[HIGHEST_LOWEST] = 2;
  

  
  for (int i = 2; i < n; i++)
  {
    dp[CURRENT_CURRENT] = INF;
    if (a[i - 2] < a[i - 1]  && a[i - 1] > a[i])
      dp[CURRENT_CURRENT] = min(dp2[CURRENT_CURRENT], dp[CURRENT_CURRENT]);
    if (a[i - 2] > a[i - 1]  && a[i - 1] < a[i])
      dp[CURRENT_CURRENT] = min(dp2[CURRENT_CURRENT], dp[CURRENT_CURRENT]);
    
    if (a[i - 1] > a[i])
      dp[CURRENT_CURRENT] = min(dp2[LOWEST_CURRENT], dp[CURRENT_CURRENT]);
    if (a[i - 1] < a[i])
      dp[CURRENT_CURRENT] = min(dp2[HIGHEST_CURRENT], dp[CURRENT_CURRENT]);
    
    dp[CURRENT_LOWEST] = INF;
    if (a[i - 2] < a[i - 1])
      dp[CURRENT_LOWEST] = min(dp2[CURRENT_CURRENT] + 1, dp[CURRENT_LOWEST]);
    dp[CURRENT_LOWEST] = min(dp2[LOWEST_CURRENT] + 1, dp[CURRENT_LOWEST]);

    dp[CURRENT_HIGHEST] = INF;
    if (a[i - 2] > a[i - 1])
      dp[CURRENT_HIGHEST] = min(dp2[CURRENT_CURRENT] + 1, dp[CURRENT_HIGHEST]);
    dp[CURRENT_HIGHEST] = min(dp2[HIGHEST_CURRENT] + 1, dp[CURRENT_HIGHEST]);

    dp[LOWEST_CURRENT] = INF;
    dp[LOWEST_CURRENT] = min(dp2[CURRENT_LOWEST], dp[LOWEST_CURRENT]);
    dp[LOWEST_CURRENT] = min(dp2[HIGHEST_LOWEST], dp[LOWEST_CURRENT]);

    dp[LOWEST_HIGHEST] = INF;
    dp[LOWEST_HIGHEST] = min(dp2[CURRENT_LOWEST] + 1, dp[LOWEST_HIGHEST]);
    dp[LOWEST_HIGHEST] = min(dp2[HIGHEST_LOWEST] + 1, dp[LOWEST_HIGHEST]);

    dp[HIGHEST_CURRENT] = INF;
    dp[HIGHEST_CURRENT] = min(dp2[CURRENT_HIGHEST], dp[HIGHEST_CURRENT]);
    dp[HIGHEST_CURRENT] = min(dp2[LOWEST_HIGHEST], dp[HIGHEST_CURRENT]);

    dp[HIGHEST_LOWEST] = INF;
    dp[HIGHEST_LOWEST] = min(dp2[CURRENT_HIGHEST] + 1, dp[HIGHEST_LOWEST]);
    dp[HIGHEST_LOWEST] = min(dp2[LOWEST_HIGHEST] + 1, dp[HIGHEST_LOWEST]);
    
      
    for (int j = 0; j < 7; j++) dp2[j] = dp[j];
  }
  
  int ret = INF;
  for (int i = 0; i < 7; i++)
  {
    ret = min(ret, dp[i]);
  }


  int m1 = 0;
  for (int i = 0; i + 1 < n; i++)
  {
    if (i % 2 == 0)
    {
      if (a[i] >= a[i + 1])
      {
        a[i + 1] = 1000000000;
        m1++;
      }
    }
    else
    {
      if (a[i] <= a[i + 1])
      {
        a[i + 1] = -1000000000;
        m1++;
      }
    }
  }
  
  int m2 = 0;
  for (int i = 0; i + 1 < n; i++)
  {
    if (i % 2 == 0)
    {
      if (b[i] <= b[i + 1])
      {
        b[i + 1] = -1000000000;
        m2++;
      }
    }
    else
    {
      if (b[i] >= b[i + 1])
      {
        b[i + 1] = 1000000000;
        m2++;
      }
    }
  }
  
  int ret2 = min(m1, m2);
  
  //assert(ret == ret2);
  
  ret = min(ret, ret2);

  printf("%d\n", ret);
  
  //printf("%lld\n", clock() - t);
  return 0;
}