// Source: https://usaco.guide/general/io
#include <bits/stdc++.h>
using namespace std;
int N;
string line;
bool ok(int mid) {
int p = -1, a = -1;
int pUsed = -1;
for (int i = 0; i < N; i++) {
if (line[i] == '*') {
if (a == -1) {
if (p != -1) {
bool cover;
if (pUsed > 0) {
int pToUse = i - p;
int minUse = min(pUsed, pToUse);
int maxUse = max(pUsed, pToUse);
if (2*minUse+maxUse <= mid) cover = true;
else cover = false;
} else {
cover = (i-p <= mid);
}
if (!cover) {
p = -1;
pUsed = -1;
a = i;
}
} else {
a = i;
}
}
} else if (line[i] == 'P') {
if (p == -1) {
p = i;
if (a != -1) {
if (i - a > mid) return false;
pUsed = (i-a);
if (mid-pUsed <= 1) {
p = -1;
pUsed = -1;
}
a = -1;
} else {
pUsed = -1;
}
} else {
p = i;
pUsed = -1;
}
}
//cout << mid << ", " << i << ": " << p << "/" << a << "/" << pUsed << endl;
}
if (a > 0) return false;
return true;
}
int main() {
cin >> N >> line;
int lo = 0, hi = N*2;
while (lo < hi) {
int mid = lo + (hi-lo)/2;
if (ok(mid)) {
hi = mid;
} else {
lo = mid+1;
}
}
cout << lo << endl;
}
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