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

enum inf = 1_001_001_001;
enum inf6 = 1_001_001_001_001_001_001L;
enum mod = 1_000_000_007L;


void main() {
    int N;
    scan(N);
    auto adj = new int[][](N*N, 0);
    auto deg = new int[](N*N);
    foreach (i ; 0 .. N) {
        auto a = readln.split.to!(int[]);
        a[] -= 1;
        foreach (j ; 0 .. N - 2) {
            int x = i, y = a[j];
            if (x > y) swap(x, y);
            int nx = i, ny = a[j + 1];
            if (nx > ny) swap(nx, ny);

            adj[N * x + y] ~= N * nx + ny;
            deg[N * nx + ny]++;
        }
    }

    debug {
        writeln(adj);
    }

    int[] next;
    auto dp = new int[](N*N);
    foreach (i ; 0 .. N*N) {
        if (deg[i] == 0) next ~= i;
    }

    while (!next.empty) {
        int[] nn;

        foreach (v ; next) {
            foreach (u ; adj[v]) {
                chmax(dp[u], dp[v] + 1);
                deg[u]--;
                if (deg[u] == 0) nn ~= u;
            }
        }

        next = nn;
    }

    debug {
        writeln(dp);
    }

    if (deg.any!"a > 0") {
        writeln(-1);
    }
    else {
        writeln(dp.reduce!max + 1);
    }
}







void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}

void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}

bool chmin(T, U...)(ref T x, U args) {
    bool isChanged;

    foreach (arg; args) {
        if (x > arg) {
            x = arg;
            isChanged = true;
        }
    }

    return isChanged;
}

bool chmax(T, U...)(ref T x, U args) {
    bool isChanged;

    foreach (arg; args) {
        if (x < arg) {
            x = arg;
            isChanged = true;
        }
    }

    return isChanged;
}
