int check(int t) {   //check whether t is a possible time
    int i, j, now = 0, num = 0;
    
    i = t/20 + 2;
    
    while(i <= n) {
        while(i <= n && ind[i] == 0)
            i++;
        if((i-1)*4+10*num > t)
            return 0;
        j = (t-10*num+20*i+4)/24;
        i = (t-10*num+16*j+4)/20+1;
        num++;
    }
    return 1;
}   //end check