Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

C++: #include <iostream>
#include <algorithm>
#include <string>
using namespace std;

int main(void) {
  int N;
  cin >> N;

  while (N--) {
    int num_size;
    string num1, num2;
    cin >> num1 >> num2;

    reverse(num1.begin(), num1.end());
    reverse(num2.begin(), num2.end());

    num_size = max(num1.size(), num2.size());
    if (num_size > 80) {
      cout << "overflow" << endl;
      continue;
    }
    if (num1.size() > num2.size()) {
      int diff = num1.size() - num2.size();
      while (diff--) {
        num2 += '0';
      }
    } else {
      int diff = num2.size() - num1.size();
      while (diff--) {
        num1 += '0';
      }
    }

    string sum;
    bool flag = false;
    for (int i = 0; i < num_size; i++) {
      int n1 = num1[i] - '0';
      int n2 = num2[i] - '0';
      int tmp = n1 + n2;
      if (flag) {
        tmp++;
        flag = false;
      }
      if (tmp >= 10) {
        tmp %= 10;
        flag = true;
      }
      sum += tmp + '0';
    }

    if (sum.size() > 80) {
      cout << "overflow" << endl;
      continue;
    }
    
    reverse(sum.begin(), sum.end());
    cout << sum << endl;

  }

  return 0;
}

C++: #define scanf_s scanf
#define sscanf_s sscanf
#include <stdio.h>
#include <string>
#include <iostream>
#include <math.h>
using namespace std;
#define MAX 81
#define _MAX 100
//#define CONSTNUM 10000000
int main(void)
{
	int n, s = 0, x, y, r = 0, a, b;
	char sum[50][_MAX], astr[_MAX], bstr[_MAX],num[11] = {"0123456789"};
	char ca, cb;
	scanf_s("%d", &n);
	for (int i = 0; i < n; ++i) {
		scanf_s("%s %s", &astr, &bstr);
		for (x = 0; astr[x] != '\0'; ++x);
		for (y = 0; bstr[y] != '\0'; ++y);

		if (x >= 80 || y >= 80) {
			printf("overflow\n");
			continue;
		}
		for (int j = 0; j < x || j < y; ++j) {
			if (j < x && j < y) {
				ca = astr[x - j - 1]; cb = bstr[y - j - 1];
				sscanf_s(&ca, "%d", &a);
				sscanf_s(&cb, "%d", &b);
				if (x > y) sum[i][x - j] = num[(a + b + r) % 10];
				else sum[i][y - j] = num[(a + b + r) % 10];
				if (a + b + r >= 10) r = (a + b + r) / 10;
				else r = 0;
			}
			else {
				if (x > y) { 
					if (r != 0) {
						ca = astr[x - j - 1];
						sscanf_s(&ca, "%d", &a);
						sum[i][x - j] = num[(a + r) % 10];
						if (a + r >= 10) r = (a + r) / 10;
						else r = 0;
					}
					else {
						sum[i][x - j] = astr[x - j - 1];
					}
				}
				else { 
					if (r != 0) {
						cb = bstr[y - j - 1];
						sscanf_s(&cb, "%d", &b);
						sum[i][y - j] = num[(b + r) % 10];
						if (b + r >= 10) r = (b + r) / 10;
						else r = 0;
					}
					else {
						sum[i][y - j] = astr[y - j - 1];
					}
				}
			}
		}
		if (y <= 80 - r || x <= 80 - r) {
			if(r == 1) 	printf("%d",r);
			for (int j = 1 ; j <= x  || j <= y; ++j) {
				printf("%c", sum[i][j]);
			}
		}
		else {
			printf("overflow");
		}
		printf("\n");
		r = 0;
	}
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<stack>
#include<queue>
#include<cstdio>
#include<climits>
#include<cmath>
#include<cstring>
#include<string>
#include<complex>
#include<iomanip>

#define EPS (1e-10)
using namespace std;
#define PI (3.1415926535)

int main(){
	string a, b;
	vector<int> m, n, s;
	int set;

	m.resize(81, 0);
	n.resize(81, 0);
	s.resize(81, 0);

	cin >> set;
	while(set-- > 0){
		cin >> a >> b;
		for(int i = 0; i < 81; i++){
			m[i] = n[i] = s[i] = 0;
		}		
		if(a.size() > 80 || b.size() > 80){
			cout << "overflow" << endl;
		}else{
			reverse(a.begin(), a.end());
			reverse(b.begin(), b.end());
			for(int i = 0; i < a.size(); i++){
				m[i] = a[i] - '0';
			}
			for(int i = 0; i < b.size(); i++){
				n[i] = b[i] - '0';
			}
			for(int i = 0; i < 80; i++){
				s[i] += m[i] + n[i];
				if(s[i] >= 10){
					s[i+1]++;
					s[i]-=10;
				}
			}
			int digit = 81;
			while(--digit > 0){
				if(s[digit] != 0){
					break;
				}
			}
			if(digit >= 79){
				cout << "overflow" << endl;
			}else{
				for(int i = digit; i >= 0; i--){
					cout << s[i];
				}
				cout << endl;
			}
		}
	}

	return 0;
}