POJ 2891 Strange Way to Express Integers

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?

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 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.

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

#include <iostream>
#include <cstring>
#include <cmath>
#include <cstdio>
#define N 1000000
using namespace std;
struct num
{
    __int64 a,r;
}t[N];
__int64 x,y;
int main()
{
    //freopen("data.in","r",stdin);
    __int64 f(__int64 a,__int64 b);
    __int64 a,r;
    int n;
    while(scanf("%d",&n)!=EOF)
    {
        for(int i=0;i<=n-1;i++)
        {
            scanf("%I64d %I64d",&t[i].a,&t[i].r);
        }
        __int64 X=1,Y=0;
        bool ch = true;
        __int64 ans;
        for(int i=0;i<=n-1;i++)
        {
            __int64 ai = t[i].a;
            __int64 ri = t[i].r;
            ri = ((ri-Y)%ai+ai)%ai;
            __int64 d = f(X,ai);
            if(ri%d!=0)
            {
                ch = false;
                break;
            }
            __int64 temp= ai/d;
            __int64 u = (((-x/temp)+1)*temp)+x;
            if((x>0&&u<x)||x<0)
            {
                x = u;
            }
            temp = ai/d;
            __int64 v=(x*(ri/d))%(temp);
            __int64 pre = X;
            X = (X*temp);
            Y = pre*v+Y;
            if(i==n-1)
            {
                ans = Y%X;
            }
        }
        if(ch)
        {
            printf("%I64d\n",ans);
        }else
        {
            printf("-1\n");
        }
    }
    return 0;
}
__int64 f(__int64 a,__int64 b)
{
    if(b==0)
    {
        x = 1;
        y = 0;
        return a;
    }
    __int64 d = f(b,a%b);
    __int64 temp = x;
    x = y;
    y = temp - (a/b)*y;
    return d;
}