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 <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>

using namespace std;

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 11;
const int MAX_LEN = 8;
char inputA[MAX_INPUT];
char inputB[MAX_INPUT];

void toBigInt(char *str, long *r) {
    int len = strlen(str);
    int endIndex = ARRAY_SIZE - (len / MAX_LEN) - 1;

    {
        int offset = 0;
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            char temp[MAX_LEN + 1] = { 0 };
            int readLen = ((len - offset) < MAX_LEN) ? len - offset : MAX_LEN;
            char* start = str + len - offset - readLen;

            strncpy(temp, start, readLen);
            r[ARRAY_SIZE - i- 1] = (long)atoi(temp);
            offset += readLen;
        }
    }
}

int main() {
    const char OVER[] = "overflow\n";
    int n = 0;
    scanf("%d", &n);

    for (int i = 0; i < n; ++i) {
        scanf("%s\n%s", &inputA, &inputB);
        long a[ARRAY_SIZE] = { 0 };
        long b[ARRAY_SIZE] = { 0 };
        long r[12] = { 0 };
        int lenA = 0;
        int lenB = 0;
        const int MAX_INPUT_LEN = 80;
        lenA = strlen(&inputA[0]);
        lenB = strlen(&inputB[0]);
        if ((MAX_INPUT_LEN < lenA) || (MAX_INPUT_LEN < lenB)) {
            printf(OVER);
            continue;
        }

        toBigInt(inputA, a);
        toBigInt(inputB, b);
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            int curRIndex = 12 - i - 1;
            r[curRIndex] += a[ARRAY_SIZE - i - 1] + b[ARRAY_SIZE - i - 1];
            const int MAX = 99999999;
            if (MAX < r[curRIndex]) {
                r[curRIndex - 1] = r[curRIndex] / (MAX + 1);
                r[curRIndex] %= (MAX + 1);
            }
        }

        if ((9999999 < r[2]) || (0 < r[1])) {
            printf(OVER);
            continue;
        }

        {
            bool started = false;
            bool noOutput = true;
            for (int i = 0; i < 12; ++i) {
                if (!started && (0 < r[i])) {
                    started = true;
                    printf("%d", r[i]);
                    noOutput = false;
                    continue;
                }

                if (started) {
                    long temp = (r[i] == 0) ? 1 : r[i];
                    for (int j = 0; temp * pow(10, j) < 10000000; ++j) {
                        printf("0");
                    }

                    printf("%d", r[i]);
                }
            }

            if (noOutput) {
                printf("0");
            }
        }
        printf("\n");

        for (int i = 0; i < MAX_INPUT; ++i) {
            inputA[i] = 0;
            inputB[i] = 0;
        }
    }

    return 0;
}

C++: #include <iterator>
#include <typeinfo>
#include <list>
#include <map>     
#include <numeric>
#include <utility>
#include <queue>
#include <set>
#include <sstream>
#include <iomanip>
#include <iostream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define ll long long
#define ld long double
#define rep(a,t) for(int a=0;a<t;++a) 
#define forever while(true)
#define Sort(a) sort(a.begin(),a.end())
#define Reverse(a) reverse(a.begin(),a.end())
#define pb push_back
#define int_maxvalue numeric_limits<int>::max()
ll mod = 1e9 + 7;
string Addition(string a,string b) {
	string ans = "";
	int bef = 0;
	int be1 = 0;
	for (int i = a.length() - 1; i >= 0; i--) {
		int added = stoi(a.substr(i, 1)) + stoi(b.substr(i, 1))+be1;

		if (added >= 10) {
			bef = 1;
			added -= 10;
		}
		else {
			bef = 0;
		}

		ans = to_string(added)+ans;
		/*
		stringstream ss;
		ss << added;

		ans = ss.str() + ans;*/
		if (i == 0) 
		{
			break;
		}
		be1 = bef;
	}
	if (be1 == 1) {
		ans = "1" + ans;
	}
	return ans;
}
int main() {
	int n;
	cin >> n;
	rep(o1, n) {
		string a, b;
		cin >> a >> b;
		if (a.length() < b.length()) {
			swap(a, b);
		}
		string b1 = b;
		for (int i = 0; i < a.length() - b1.length(); ++i) {
			b = '0' + b;
		}
		string ans = Addition(a, b);
		if (ans.length() > 80) { cout << "overflow" << endl; }
		else {
			cout << ans << endl;
		}
	}
	return 0;
}

C++: #include <cstdio>
#include <cstring>
using namespace std;

int main() {
  int n;
  char f[110], s[110];
  scanf("%d", &n);
  while (n--) {
    scanf("%s%s", f, s);
    int fp = strlen(f);
    int sp = strlen(s);
    if (fp > 80 || sp > 80) {
      puts("overflow");
      continue;
    }
    int c = 0, p = 0, ans[90];
    while (fp > 0 || sp > 0) {
      int a = 0, b = 0;
      if (fp > 0) a = f[--fp] - '0';
      if (sp > 0) b = s[--sp] - '0';
      ans[p++] = (a + b + c) % 10;
      c = (a + b + c) / 10;
    }
    if (c) ans[p] = c;
    else --p;
    if (p >= 80) {
      puts("overflow");
      continue;
    }
    for (; p >= 0; --p) {
      printf("%d", ans[p]);
    }
    puts("");
  }
}