#include <bits/stdc++.h>
typedef long long ll;
#define f first
#define s second
#define pb push_back
#define ep emplace
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define uniquev(v) sort(all(v)), (v).resize(unique(all(v)) - (v).begin())
#define mem(f,x) memset(f , x , sizeof(f))
#define sz(x) (int)(x).size()
#define __lcm(a, b) (1ll * ((a) / __gcd((a), (b))) * (b))
#define mxx *max_element
#define mnn *min_element
#define cntbit(x) __builtin_popcountll(x)
using namespace std;
const int N = 4e5 + 100;
const int M = 2e5;
int p[N], pr[N];
vector <int> pos[N];
int solve() {
int n;
cin >> n;
string s;
cin >> s;
s = '0' + s;
p[0] = M;
pos[M].pb(0);
for (int i = 1; i <= n; i++) {
if (s[i] == 'J') {
p[i]++;
}
if (s[i] == 'O') {
p[i]--;
}
if (s[i] == 'I') {
pr[i]++;
}
pr[i] += pr[i - 1];
p[i] += p[i - 1];
pos[p[i]].pb(i);
}
int ans = 0;
for (int i = 0; i < N; i++) {
if (pos[i].empty())
continue;
map <int, int> mp;
for (auto x : pos[i]) {
int cur = x - pr[x] * 3;
if (mp.find(cur) != mp.end()) {
ans = max(ans, x - mp[cur]);
} else {
mp[cur] = x;
}
}
}
return ans;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
//cin >> t;
while (t--) {
cout << solve() << '\n';
}
}
