int ax = 0; int ay = 0; // alice's x and y coordinates
int bx = r; int by = s; // bob's x and y coordinates
int t = 0; // keep track of the current time
while(ax < bx && ay < by){
    // every second, update alice's and bob's coordinates and the time
    ax += m; ay += n;
    bx -= p; by -= q;
    t++;
}
if(ax == bx && ay == by){ // if they are in the same location
    pw.println(t); // they meet at time t
} else {
    pw.println(-1); // they never meet
}
pw.close(); // flush the output