题意：
给你T组数据，每组有x,y,p,q四个数，x/y是你当前提交正确率，让你求出最少需要再提交几次可以达到目标正确率p/q;
思路：
假设提交B次，正确A次，那么可以得到（x+A）/(y+B)=p/q,可以推出x+A=k*p,y+B=k*q.那么A=k*p-x，B=K*q-y;
这样我们只需要二分枚举k，判断A,B是否满足（0<=A<=B）即可。
代码：
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn=1e6+10;
const int mod=1e9;
int main()
{int t;scanf("%d",&t);while(t--){ll x,y,p,q;ll ans=-1;scanf("%lld%lld%lld%lld",&x,&y,&p,&q);ll l=0,r=1000000000;while(r-l>=0){ll mid,b,d;mid=(l+r)/2;b=mid*p-x;d=mid*q-y;if(b>=0&&d>=0&&b<=d){ans=mid;r=mid-1;}elsel=mid+1;}if(ans==-1)printf("-1\n");elseprintf("%lld\n",ans*q-y);}return 0;
}题目：
You are an experienced Codeforces user. Today you found out that during your activity on Codeforces you have made y submissions, out of which x have been successful. Thus, your current success rate on Codeforces is equal to x?/?y.
Your favorite rational number in the [0;1] range is p?/?q. Now you wonder: what is the smallest number of submissions you have to make if you want your success rate to be p?/?q?
Input
The first line contains a single integer t (1?≤?t?≤?1000) — the number of test cases.
Each of the next t lines contains four integers x, y, p and q (0?≤?x?≤?y?≤?109; 0?≤?p?≤?q?≤?109; y?>?0; q?>?0).
It is guaranteed that p?/?q is an irreducible fraction.
Hacks. For hacks, an additional constraint of t?≤?5 must be met.
Output
For each test case, output a single integer equal to the smallest number of submissions you have to make if you want your success rate to be equal to your favorite rational number, or -1 if this is impossible to achieve.
Example
Input
4 3 10 1 2 7 14 3 8 20 70 2 7 5 6 1 1
Output
4 10 0 -1
Note
In the first example, you have to make 4 successful submissions. Your success rate will be equal to 7?/?14, or 1?/?2.
In the second example, you have to make 2 successful and 8 unsuccessful submissions. Your success rate will be equal to 9?/?24, or 3?/?8.
In the third example, there is no need to make any new submissions. Your success rate is already equal to 20?/?70, or 2?/?7.
In the fourth example, the only unsuccessful submission breaks your hopes of having the success rate equal to 1.