Флойд

#include <bits/stdc++.h>

using namespace std;

const int N = 105;
const int inf = 1e9;

int n;
int a[N][N], f[N][N];

int main(){
    cin >> n;

    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            cin >> a[i][j];
            if (a[i][j] == -1) {
                f[i][j] = inf;
            } else {
                f[i][j] = a[i][j];
            }
        }
    }

    for (int k = 1; k <= n; k++) {
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= n; j++) {
                f[i][j] = min(f[i][j], f[i][k] + f[k][j]);
            }
        }
    }

    int diameter = 0;

    for (int s = 1; s <= n; s++) {
        for (int t = 1; t <= n; t++) {
            if (f[s][t] == inf) continue;
            diameter = max(diameter, f[s][t]);
        }
    }

    cout << diameter << endl;
}