# | Time | Username | Problem | Language | Result | Execution time | Memory |
---|---|---|---|---|---|---|---|
19470 | nosiar | Σ (kriii4_P2) | C++98 | 0 ms | 0 KiB |
This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <iostream>
#include <tuple>
using namespace std;
long long mod = 1000000007;
long long m;
long long n[10000];
long long s[10000];
tuple<long long, long long, long long> extended_gcd(long long a, long long b)
{
if (b == 0)
return tuple<long long, long long, long long> { a, 1, 0 };
auto r = extended_gcd(b, a % b);
long long d = get<0>(r);
long long x = get<1>(r);
long long y = get<2>(r);
return tuple<long long, long long, long long>{ d, y, x - a/b*y };
}
/* a has a (unique) multiplicative inverse iif gcd(a,n)=1 */
long long inverse(long long a)
{
auto r = extended_gcd(a, mod);
if (get<0>(r) != 1) return -1;
return get<1>(r);
}
int main() {
cin >> m;
long long ans = 0;
for(int i = 0; i < m; ++i)
cin>>n[i]>>s[i];
long long a=0,b=1;
for(int i = 0; i < m; ++i)
b = (b*n[i])%mod;
for(int i = 0; i < m; ++i)
a = (a + ((s[i] * b % mod) * inverse(n[i]) %mod))%mod;
cout << (a*inverse(b))%mod << endl;
return 0;
}