POJ2891 Strange Way to Express Integers [中国剩余定理]

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
typedef long long ll;
inline ll read(){
char c=getchar();ll x=0,f=1;
while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}
while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();}
return x*f;
}
ll n,a1,m1,a2,m2;
void exgcd(ll a,ll b,ll &d,ll &x,ll &y){
if(b==0) d=a,x=1,y=0;
else exgcd(b,a%b,d,y,x),y-=(a/b)*x;
}
int main(){
freopen("in","r",stdin);
while(scanf("%lld",&n)!=EOF){
int flag=0;
m1=read();a1=read();n--;
while(n--){
m2=read();a2=read();
if(flag) continue;
ll d,t1,t2;
exgcd(m1,m2,d,t1,t2);
if((a2-a1)%d){flag=1;continue;}
t1*=(a2-a1)/d;
m2/=d;
t1=(t1%m2+m2)%m2;
a1=a1+m1*t1;
m1*=m2;
}
if(flag) puts("-1");
else printf("%lld\n",a1);
}
}