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 <string>
#include <cstdio>
#include <algorithm>
#include <stdexcept>

class biglong {
	static const int part_digits = 8;
	static const int max_parts = 10;
	static const int max_digits = part_digits * max_parts;
	static const unsigned long max_part_value = 100000000;
	unsigned long parts[max_parts];
public:
	biglong(unsigned long value) : parts() { parts[0] = value; }
	biglong(const biglong& rhs) {
		std::copy(rhs.parts, rhs.parts + max_parts, parts);
	}
	explicit biglong(const std::string& rhs) : parts() {
		if (rhs.size() > max_digits) {
			throw std::overflow_error("biglong overflow");
		}
		int part_count = (static_cast<int>(rhs.size()) + part_digits - 1) / part_digits;
		int pos = static_cast<int>(rhs.size());
		for (int i = 0; i < part_count; ++i) {
			pos -= part_digits;
			if (pos < 0) {
				parts[i] = std::atol(rhs.substr(0, pos + part_digits).c_str());
			} else {
				parts[i] = std::atol(rhs.substr(pos, part_digits).c_str());
			}
		}
	}
	biglong& operator+=(const biglong& rhs) {
		for (int i = 0; i < max_parts - 1; ++i) {
			parts[i] += rhs.parts[i];
			if (parts[i] >= max_part_value) {
				parts[i] -= max_part_value;
				++parts[i + 1];
			}
		}
		if (parts[max_parts - 1] >= max_part_value) {
			throw std::overflow_error("biglong overflow");
		}
		return *this;
	}
	std::string c_str() const {
		std::string result;
		for (int i = max_parts - 1; i >= 0; --i) {
			if (!result.empty() || parts[i] != 0) {
				char buff[12] = {};
				if (result.empty()) {
					sprintf(buff, "%lu", parts[i]);
				} else {
					sprintf(buff, "%0*lu", part_digits, parts[i]);
				}
				result += buff;
			}
		}
		if (result.empty()) {
			result = "0";
		}
		return result;
	}
};

biglong operator+(const biglong& lhs, const biglong& rhs)
{
	return biglong(lhs) += rhs;
}

std::ostream& operator<<(std::ostream& os, const biglong& value)
{
	return os << value.c_str();
}

int main()
{
	int count = 0;
	std::cin >> count;
	for (int i = 0; i < count; ++i) {
		std::string input1, input2;
		std::cin >> input1 >> input2;
		try {
			biglong value1(input1);
			biglong value2(input2);
			biglong sum = value1 + value2;
			std::cout << sum << std::endl;
		}
		catch (const std::overflow_error&) {
			std::cout << "overflow" << std::endl;
		}
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

template<class T> inline T sqr(T x) {return x*x;}

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef unsigned long long ULL;

#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

const double EPS = 1e-10;
const double PI  = acos(-1.0);

#define CLR(a) memset((a), 0 ,sizeof(a))

#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, -1, 0, 1};

int main() {
  int n;
  cin >> n;

  REP(i, n) {
    char s1[100], s2[100], ans[100];
    cin >> s1 >> s2;

    int c = 0;
    int cnt = 0, j = strlen(s1)-1 , k = strlen(s2)-1;
    while(j >= 0 || k >= 0 || c) {
      char s = c;
      if(j >= 0) s += s1[j] - '0';
      if(k >= 0) s += s2[k] - '0';
      if(s > 9) {
	s -= 10;
	c = 1;
      }
      else 
	c = 0;

      s += '0';

      ans[cnt] = s;
      cnt++, j--, k--;
    }

    int len = strlen(ans);
    if(len > 80) 
      cout << "overflow" << endl;
    else {
      for(j = len - 1; j >= 0; j--) {
	cout << ans[j];
      }
      cout << endl;
    }
  }
}

C++: #include <cstdio>
#include <cstring>
char A[81],B[81],C[84];
int main() {
	int N,Alen,Blen;
	scanf("%d",&N);
	for(int i=0;i<N;i++) {
		scanf("%s",A);
		scanf("%s",B);
		Alen=strlen(A);
		Blen=strlen(B);
		memset(C,-1,sizeof(C));
		for(int j=Alen-1,k=0;j>=0;j--,k++) {
			C[k]=A[j]-'0';
		}
		for(int j=Blen-1,k=0;j>=0;j--,k++) {
			C[k]+=B[j]-'0';
		}
		for(int j=0;C[j]!=-1;j++) {
			if(C[j]/10) {
				if(C[j+1]!=-1)C[j+1]+=C[j]/10;
				else C[j+1]=C[j]/10;
			}
			C[j]%=10;
		}
		for(int j=82;j>=0;j--) {
			if(C[j]!=-1) {
				if(j>80) {
					printf("overflow");
					break;
				}
				putchar('0'+C[j]);
			}
		}
		putchar('\n');
	}
}