#include<bits/stdc++.h>

 #define lowbit(x) x&(-x)

 #define LL long long

 #define N 100005

 #define M 1000005

 #define mod 2147483648LL

 #define inf 0x7ffffffff

 using namespace std;

 inline int ra()

 {

     int x=,f=; char ch=getchar();

     while (ch<'' || ch>''){if (ch=='-') f=-; ch=getchar();}

     while (ch>='' && ch<=''){x=x*+ch-''; ch=getchar();}

     return x*f;

 }

 int n,k,last,ans,cnt;

 int a[N<<],b[N<<];

 int main()

 {

     n=ra(); k=ra();

     for (int i=; i<=n; i++) a[i]=ra();

     int last,start=n+;

     for (int i=; i<=n; i++)

         if (a[i]<) {

             start=i;

             break;

         }

     if (start<=n) ans=,k--;

     else {

         cout<<"";

         return ;

     }

     for (int i=start+; i<=n; i++)

     {

         if (a[i]<) k--;

         if (a[i]>= && a[i-]<) last=i,ans++;

         if (a[i]>= && a[i+]<) b[++cnt]=i-last+,ans++;

     }

     if (k<)

     {

         cout<<"-1";

         return ;

     }

     sort(b+,b+cnt+);

     for (int i=; i<=cnt; i++)

         if (k-b[i]<) break;

             else k-=b[i],ans-=;

     if (a[n]>=)

         for (int i=n; i>=; i--)

             if (a[i]<)

             {

                 b[++cnt]=n-i;

                 break;

             }

     if (b[cnt]<=k && cnt) ans--;

     cout<<ans;

     return ;

 }