POJ 2891 Strange Way to Express Integers(一元线性同余方程组)

Strange Way to Express Integers

Time Limit: 1000MS Memory Limit: 131072KTotal Submissions: 14168 Accepted: 4590

Description

Elina is reading a book written by Rujia Liu, which introduces a strange way to express non-negative integers. The way is described as following:

Choose k different positive integers a₁, a₂, …, a_k. For some non-negative m, divide it by every a_i (1 ≤ i ≤ k) to find the remainder r_i. If a₁, a₂, …, a_k are properly chosen, m can be determined, then the pairs (a_i, r_i) can be used to express m.

“It is easy to calculate the pairs from m, ” said Elina. “But how can I find m from the pairs?”

Since Elina is new to programming, this problem is too difficult for her. Can you help her?

Input

The input contains multiple test cases. Each test cases consists of some lines.

• Line 1: Contains the integer k.
• Lines 2 ~ k + 1: Each contains a pair of integers a_i, r_i (1 ≤ i ≤ k).

Output

Output the non-negative integer m on a separate line for each test case. If there are multiple possible values, output the smallest one. If there are no possible values, output -1.

Sample Input

28 711 9

Sample Output

31

Hint

All integers in the input and the output are non-negative and can be represented by 64-bit integral types.

Source

总结：

不断的合并两个同余方程组，知道接出结果

////  main.cpp//  Strange Way to Express Integers////  Created by 张嘉韬 on 16/8/24.//  Copyright © 2016年 张嘉韬. All rights reserved.//#include <iostream>#include <cstring>#include <cstdio>using namespace std;typedef long long ll;void exgcd(ll a,ll b,ll &d,ll&x,ll&y){    if(!b) {d=a,x=1,y=0;}    else exgcd(b,a%b,d,y,x),y-=x*(a/b);}int main(int argc, const char * argv[]) {    //freopen("/Users/zhangjiatao/Documents/暑期训练/input.txt","r",stdin);    ll k;    while(scanf("%lld",&k)!=EOF)    {        ll r1,a1;        int flag=1;        scanf("%lld%lld",&a1,&r1);        for(int i=1;i<=k-1;i++)        {            ll r2,a2;            scanf("%lld%lld",&a2,&r2);            if(flag)            {                ll a,b,c,d,x0,y0;                a=a1,b=a2,c=r2-r1;                exgcd(a,b,d,x0,y0);                if(c%d!=0) flag=0;                ll t=b/d;                x0=(x0*(c/d)%t+t)%t;                r1+=a1*x0;                a1*=a2/d;            }        }        if(!flag)        {            printf("-1\n");        }        else            printf("%lld\n",r1);            }    return 0;}