void pop(si &s1, si &s2){
if(!s2.empty()){
s2.pop();
return;
}
if(s1.empty())
return;
while(true){
int i=s1.top();
s1.pop();
if(s1.empty())
return;
s2.push(i);
}
}
int top(si &s1, si& s2){
if(!s2.empty())
return s2.top();
if(s1.empty())
return -1;
while(!s1.empty()){
s2.push(s1.top());
s1.pop();
}
return s2.top();
}