while(!q.empty()) {
        int cur = q.front();
        q.pop();
        for(int i = 0; i < n; i++) {
            if(a[cur][i] != -1) {
                d[i] = min(d[i], d[cur] + a[cur][i]);
                if(!used[i]) {
                    d[i] = min(d[i], d[cur] + a[cur][i]);
                    q.push(i);
                    used[i] = 1;
                }
            }
        }
    }