#include<cstdio>

 #include<algorithm>

 #include<cstring>

 #include<vector>

 using namespace std;

 #define fi first

 #define se second

 #define mp make_pair

 #define pb push_back

 typedef long long ll;

 typedef unsigned long long ull;

 typedef pair<int,int> pii;

 int a[],ans[],pre[];

 int n;

 int main()

 {

     int i;

     scanf("%d",&n);

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

         scanf("%1d",&a[i]);

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

         pre[i]=a[i]==?i:pre[i-];

     int pre2=;

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

     {

         if(i>=&&a[i]==&&a[i-]==&&a[i-]==)    pre2=i;

         ans[i]=ans[i-];

         if(i>=&&a[i-]==&&a[i-]==&&a[i]==)

         {

             ans[i]=max(ans[i],ans[pre[i-]]+(i--pre[i-]));

             ans[i]=max(ans[i],ans[pre[i-]+]+(i--(pre[i-]+)));

         }

         if(pre2>pre[i])

         {

             ans[i]=max(ans[i],ans[pre2-]+(i-pre2+));

         }

         //printf("1t%d %d\n",i,ans[i]);

     }

     printf("%d",ans[n]);

     return ;

 }