C :

#include <stdio.h>
#include <math.h>
#define M 10000000 
int main() {
	int array[M] = {0};
	double a, x, s;
	long n, i, j, t, y;
	scanf("%ld", &n);
	for (i = 1; i <= n; i++) {
		scanf("%lf%ld", &a, &t);
		x = 0;
		for (j = 1; j <= t; j++) {
			x += a;
			modf(x, &s);
			y = (double)s;
			array[y] = !array[y];
		}
	}
	for (j = 1; j < M; j++) {
		if (array[j]) {
			break;
		}
	}
	printf("%d\n", j);
	return 0;
}

C++ :

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
int n,len=0;
long long a[2000010];
void init();
void work();
int main()
{
	//freopen("light.in","r",stdin);
	//freopen("light.out","w",stdout);
	init();		
	work();
	return 0;
}
void init()
{
	memset(a,0,sizeof(a));
	cin>>n;
	double s;
	int t,k;
	for(int i=1;i<=n;i++)
	{
		cin>>s>>t;	
		for(int j=1;j<=t;j++)
		{
			k=floor(s*j);
			len++;
			a[len]=k;
		}
	}
}
void work()
{
	sort(a+1,a+len+1);
	//cout<<len<<endl;	
	//for(int i=1;i<=len;i++)cout<<a[i]<<' ';
	int i=1;
	while(i<=len)
	{
		int k=1;
		while(a[i]==a[i+1]&& i<len)
		{
			i++;
			k++;
		} 
		if(k%2!=0)
		{
			cout<<a[i]<<endl;
			break;
		}
		i++;
	}
}