POJ 2891 Strange Way to Express Integers

 1 #include <cstdio>
 2 #include <cstring>
 3 
 4 using namespace std;
 5 const int N = 1005;
 6 
 7 #define ll long long
 8 ll a[N] , b[N];
 9 
10 ll ex_gcd(ll a , ll &x , ll b , ll &y)
11 {
12     if(b == 0){
13         x = 1 , y = 0;
14         return a;
15     }
16     ll ans = ex_gcd(b , x , a%b , y);
17     ll t = x;
18     x= y , y = t - (a/b)*y;
19     return ans;
20 }
21 
22 ll mod_line(int n)
23 {
24     ll r = b[0] , lcm = a[0] , x , y;
25     for(int i = 1 ; i<n ; i++)
26     {
27         ll del = b[i] - r;
28         ll g = ex_gcd(lcm , x , a[i] , y);
29         if(del % g != 0) return -1;
30         ll Mod = a[i] / g;
31         x = ((x*del/g % Mod) + Mod)%Mod;
32         r = r + lcm*x;
33         lcm = lcm*a[i]/g;
34         r %= lcm;
35     }
36     return r;
37 }
38 
39 int main()
40 {
41    // freopen("a.in" , "r" , stdin);
42     int n;
43     while(scanf("%d" , &n) == 1)
44     {
45         for(int i= 0 ; i<n ; i++)
46             scanf("%I64d%I64d" , a+i , b+i);
47         printf("%I64d\n" , mod_line(n));
48     }
49     return 0;
50 }