poj2891:Strange Way to Express Integers

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
long long a[100010],r[100010];
int n;
long long exgcd(long long a,long long b,long long &x,long long &y)
{
    if(b==0) return x=1,y=0,a;
    long long tmp=exgcd(b,a%b,y,x);
    y-=a/b*x;
    return tmp;
}
long long slove()
{
    long long M=a[1],R=r[1],x,y,d;
    for(int i=2;i<=n;i++)
    {
        d=exgcd(M,a[i],x,y);
        if((R-r[i])%d!=0) return -1;
        x=(R-r[i])/d*x%a[i];
        R-=x*M;
        M=M/d*a[i];
        R%=M;
    }
    return (R%M+M)%M;
}
int main()
{
    while(~scanf("%d",&n))
    {
        for(int i=1;i<=n;i++)
          scanf("%lld%lld",&a[i],&r[i]);
          printf("%lld\n",slove());
    }
    return 0;
}