POJ 2891 Strange Way to Express Integers【扩展欧几里德】【模线性方程组】

#include<stdio.h>
#include<string.h>
typedef long long ll;
ll ex_gcd(ll a,ll b,ll &x,ll &y){
    if(!b){
        x=1,y=0;
        return a;
    }
    ll ans=ex_gcd(b,a%b,y,x);
    y-=a/b*x;
    return ans;
}
int main(){
    int n;
    ll a,b,c1,c2,c,x,y,d;
    while(~scanf("%d",&n)){
        scanf("%lld%lld",&a,&c1);
        int f=1;
        for(int i=1;i<n;i++){
            scanf("%lld%lld",&b,&c2);
            d=ex_gcd(a,b,x,y);
            c=c2-c1;
            if(c%d)    f=0;
            c/=d;ll t=b/d;
            x=((x*c)%t+t)%t;
            c1+=a*x;//X
            a=a/d*b;//lcm
            c1%=a;
        }
        if(c1<0)    c1+=a;
        if(f)    printf("%lld\n",c1);
        else    printf("-1\n");
    }
    return 0;
}