#include<map>

 #include<cstdio>

 #include<cctype>

 inline int getint() {

     register char ch;

     while(!isdigit(ch=getchar()));

     register int x=ch^'';

     while(isdigit(ch=getchar())) x=(((x<<)+x)<<)+(ch^'');

     return x;

 }

 inline char getalpha() {

     register char ch;

     while(!isalpha(ch=getchar()));

     return ch;

 }

 std::map<std::pair<int,int>,int> map;

 int main() {

     const int n=getint();

     int ans=;

     map[std::make_pair(,)]=;

     for(register int i=,s[]={,,};i<=n;i++) {

         char ch=getalpha();

         if(ch=='J') s[]++;

         if(ch=='O') s[]++;

         if(ch=='I') s[]++;

         std::pair<int,int> p=std::make_pair(s[]-s[],s[]-s[]);

         if(map.count(p)) {

             ans=std::max(ans,i-map[p]);

         } else {

             map[p]=i;

         }

     }

     printf("%d\n",ans);

     return ;

 }