#include <iostream>
#include <vector>

using namespace std;

int main()
{
    int n;
    cin >> n;

    int a;
    int b;
    long double c=0;
    for (unsigned int i = 0; i < n; i++)
    {
        cin >> a;
        cin >> b;

        if (i != 0 )
        {
            c = c * 10 + a + b;
        }
        else
            c = a + b;


    }


    cout <<c<< endl;


}

3
1 1
9 0
9 1

try this
this test helped me

My code's producing 300. It's right, isn't it?

What is double mantissa size? About 17 decimal digits?
What is required number size? About 1,000,000 digits?

So, how your solution is supposed to work?

//wa on test 2 .. whats the problem with my code
#include<bits/stdc++.h>
using namespace std;
int main()
{
    long n,i,j=0;
    cin>>n;
    int a[2000003],b=0;
    string s;
    n*=2;
    for(i=0;i<n;i++){
        cin>>a[i];
    }
    for(i=n-1;i>=0;i=i-2){
            b=(b+a[i]+a[i-1]);
        s[j]=(b%10)+48;
        b=b/10;
        j++;
    }
    for(i=j-1;i>=0;i--){
        cout<<s[i];
    }
}