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.stdlib, std.datetime;


void main() {
    auto N = readln.chomp.to!int;
    auto A = readln.split.map!(to!int).array;
    auto B = A.split(2);
    int[][] C;

    foreach (b; B) {
        if (b.empty) continue;
        C ~= new int[](2);
        C.back[0] = b.count(0).to!int;
        C.back[1] = b.count(1).to!int;
    }

    auto M = C.length.to!int;

    auto dp = new int[][](M+1, N*2);
    foreach (i; 0..M+1) dp[i][] = 1 << 29;
    dp[0][0] = 0;

    int zero_sum = 0;
    int one_sum = 0;

    foreach (i; 0..M) {
        int zero = C[i][0];
        int one = C[i][1];
        foreach (zero_used; 0..N+1) {
            int one_used = zero_sum + one_sum - zero_used;
            int zero_amari = max(0, zero_sum - zero_used);
            dp[i+1][zero_used+zero+one] = min(dp[i+1][zero_used+zero+one], dp[i][zero_used] + max(0, one - zero_amari));
            int one_amari = max(0, one_sum - one_used);
            dp[i+1][zero_used] = min(dp[i+1][zero_used], dp[i][zero_used] + max(0, zero - one_amari));
        }
        zero_sum += C[i][0];
        one_sum += C[i][1];
    }

    auto ans = dp[M][A.count(0).to!int];
    if (ans == 1 << 29) {
        writeln(-1);
    } else {
        writeln(ans);
    }
}