import std.stdio, std.array, std.string, std.conv, std.algorithm;
import std.typecons, std.range, std.random, std.math, std.container;
import std.numeric, std.bigint, core.bitop, core.stdc.string;

void main() {
    auto N = readln.chomp.to!int;
    auto S = readln.chomp;
    auto T = S.filter!(s => s == '1' || s == '9').array;

    int ans = S.length.to!int - T.length.to!int;

    int ichi = 0;
    int kyuu = 0;

    foreach_reverse (i; 0..T.length.to!int) {
        if (T[i] == '1') {
            if (kyuu > 0) {
                ans += 1;
                kyuu -= 1;
            } else {
                ichi += 1;
            }
        } else {
            kyuu += 1;
        }
    }

    int x = min(kyuu / 2, ichi);
    ans += x;

    ans += (ichi - x) / 2;

    ans.writeln;
}