3
-1 1 -1
 4
45 -1 41 -1
 1
-1
 5
40 -1 44 46 48
 6
43 40 37 34 -1 -1
 7
-1 2 4 -1 4 1 5
 19
23 26 -1 -1 35 38 41 -1 -1 -1 53 -1 59 62 6 7 8 9 -1
 6
-1 2 6 -1 -1 6
 16
3 8 13 18 23 -1 -1 -1 43 48 53 45 -1 -1 -1 -1
 13
25 24 23 22 24 27 -1 33 -1 2 2 2 -1
 12
-1 17 -1 54 -1 64 -1 74 79 84 -1 94
 8
-1 12 14 16 18 20 -1 -1
 13
2 -1 3 1 3 1 -1 1 3 -1 -1 1 1
 5
-1 40 42 -1 46
 2
1000000000 -1
 14
-1 5 3 -1 -1 31 31 31 -1 31 -1 -1 4 7
 3
-1 1000000000 999999999
 7
32 33 34 -1 -1 37 38
 1
1000000000
 10
29 27 -1 23 42 -1 -1 45 -1 -1
 3
39 42 -1
 15
-1 28 -1 32 34 26 -1 26 -1 -1 26 26 26 -1 -1
 3
1000000000 999999999 1000000000
 17
-1 -1 -1 -1 64 68 72 -1 45 46 47 48 49 50 51 52 53
 3
-1 1 1000000000
 2
-1 1000000000
 3
999999999 -1 1000000000
 1
1
 2
1000000000 1000000000
 3
999999999 1000000000 -1
 7
11 8 5 -1 -1 -1 -1
 3
-1 1 2
 2
-1 21
 8
-1 -1 1 7 -1 9 5 2
 11
9 21 17 13 -1 -1 -1 -1 -1 -1 -1
 3
-1 1000000000 -1
 6
-1 6 1 -1 -1 -1
 1
65
 20
-1 32 37 -1 -1 -1 57 -1 -1 40 31 33 -1 -1 39 47 43 -1 35 32
 5
-1 1 7 -1 5
 18
21 19 -1 -1 -1 48 50 -1 54 -1 5 1 -1 -1 -1 37 36 35
 9
42 39 36 33 -1 -1 -1 34 39
 2
-1 -1

1
 1
 1
 1
 1
 3
 2
 2
 2
 3
 2
 1
 6
 1
 1
 3
 1
 1
 1
 2
 1
 2
 2
 2
 2
 1
 2
 1
 1
 1
 2
 2
 1
 3
 3
 1
 2
 1
 5
 2
 4
 2
 1

#include <bits/stdc++.h>
#include<ext/pb_ds/tree_policy.hpp>
#include<ext/pb_ds/assoc_container.hpp> 
#include <stdio.h> 
#include <math.h> 
#include <vector>
#include <algorithm>
#define ll long long
using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;
 
const long long MOD = 1e8+7;
int n;
int a[200100];
int ans = 0;
int preV = 0, preCnt = 0, sufCnt = 0, sufV = 0;
int i = 0; 
int diff = 0;

int main ( )
{
    scanf("%d", &n);
    for (int i = 0; i < n; i++) {
        scanf("%d", &a[i]);
    }
    
    while (i < n) {
        preV = 0, preCnt = 0, sufCnt = 0, sufV = 0;
        while (i < n && a[i] == -1) {
            i ++;
            preCnt ++;
        }
        if (i == n) {
            ans ++;
            printf("%d", ans);
            return 0;
        }
        preV = a[i++];
        while (i < n && a[i] == -1) {
            i ++;
            sufCnt ++;
        }
        if (i == n) {
            ans ++;
            printf("%d", ans);
            return 0;
        }
        sufV = a[i++];
        //printf("%d %d %d %d| %d\n", preCnt, preV, sufCnt, sufV, i);
        if (preV < sufV) {
            if ((sufV - preV) % (sufCnt+1) == 0) {
                diff = (sufV - preV) / (sufCnt+1);  
                if ((preV - 1) / diff >= preCnt) {
                    ans ++;
                    int next = sufV + diff;
                    while (i < n && (a[i] == -1 || a[i] == next)) {
                        next += diff;
                        i++;
                    }
                } else {
                    ans ++;
                    i--;
                }
            } else {
                ans ++;
                i--;
            }
        } else if (sufV == preV) {
            ans ++;
            while (i < n && (a[i] == -1 || a[i] == sufV)) {
                i++;
            }
        } else if (preV > sufV) {
            if ((preV - sufV) % (sufCnt+1) == 0) {
                ans ++;
                diff = (preV - sufV) / (sufCnt+1);
                
                int next = sufV - diff;
                while (next > 0 && i < n && a[i] == next) {
                    next -= diff;
                    i++;
                }
            } else {
                ans ++;
                i --;
            }
        }
    }
    printf("%d", ans);
    return 0;
}
