POJ 2891 Strange Way to Express Integers

#include<stdio.h>
#include<string.h>
#include<algorithm>
typedef long long LL;
int K;
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 -= x * (a / b);
}
int main()
{
while(scanf("%d", &K) == 1)
{
bool ok = true;
LL a1, r1, a2, r2;
scanf("%lld%lld", &a1, &r1);
for(int i = 1; i < K; i ++)
{
scanf("%lld%lld", &a2, &r2);
if(!ok) continue;
LL k1, k2, d;
exgcd(a1, a2, d, k1, k2);
if((r2 - r1) % d != 0) ok = false;
else
{
LL t = a2 / d;
k1 = (r2 - r1) / d % t * (k1 % t) % t;
r1 = a1 * k1 + r1, a1 = a2 / d * a1, r1 %= a1;
}
}
if(!ok) printf("-1\n");
else printf("%lld\n", (r1 % a1 + a1) % a1);
}
return 0;
}