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<stdio.h>
#include<iostream>
#include<list>
#include<map>
#include<vector>
#include<algorithm>
#include<string.h>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) range(i,0,b)
#define debug(x) cout << x << endl;
using namespace std;

int main(){
    int n,g;
    int k;
    bool check;

    cin >> n;
    rep(i,n){
        char c1[81], c2[81];
        int i1[81] = {0}, i2[81] = {0};
        int ans[82] = {0};
        check = false;
        k = 0;
        cin >> c1 >> c2;
        for(int j = strlen(c1) - 1; 0 <= j; j--, k++){
            i1[k] = c1[j] - '0';
        }
        k = 0;
        for(int j = strlen(c2) - 1; 0 <= j; j--, k++){
            i2[k] = c2[j] - '0';
        }
        rep(j, 81){
            ans[j]+= i1[j] + i2[j];
            if(ans[j] >= 10){
                ans[j]-=10;
                ans[j+1]++;
            }
        }
        if(ans[81] != 0){
            cout << "overflow";
        }else{
            for(int j = 81; 0 <= j; j--){
                if(ans[j] != 0)
                    check = true;
                if(check == true)
                    cout << ans[j];
            }
        }
        cout << endl;

    }
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <string.h>

void ctod(char *c, int *d){
	int len = strlen(c);
	char ct[10];
	int h = len / 9;
	int a = len % 9;

	for(int i=0;i<9;++i){
		d[i] = 0;
	}
	if(a != 0){
		memcpy(ct, c, a);
		ct[a] = '\0';
		d[h] = atoi(ct);
	}

	for(int i=0; i<h; ++i){
		memcpy(ct, c + len - 9*(i + 1), 9);
		ct[9] = '\0';
		d[i] = atoi(ct);
	}
}

void dtoc(char *c, int *d){
	char ct[10];
	int m = -1;
	int o = 0;
	for(int i=8; i>=0; --i){
		if(d[i] != 0){
			m = i;
			break;
		}
	}
	if(m == -1){
		c[0] = '0';
		c[1] = '\0';
		return;
	}

	sprintf(c, "%d", d[m]);
	o = strlen(c);
	for(int i = m-1; i>=0; --i){
		sprintf(c+o, "%09d", d[i]);
		o += 9;
	}
	c[o] = '\0';
}

void add(int *d1, int *d2, int *d){
	int s = 0;
	for (int i=0; i<9; ++i){
		d[i] = d1[i] + d2[i] + s;
		s = d[i] / 1000000000;
		d[i] -= 1000000000 * s;
	}
}

int main(void){
	int n = 0;
	char i1[255], i2[255], o[255];
	int d1[9], d2[9], d[9];

	scanf("%d", &n);
	for(int i=0; i<n; ++i){
		scanf("%s", i1);
		scanf("%s", i2);
		if(strlen(i1) > 80 || strlen(i2) > 80){
			puts("overflow");
			continue;
		}

		for(int j=0; j<9;++j){
			d1[j] = d2[j] = d[j] = 0;
		}
		ctod(i1, d1);
		ctod(i2, d2);
		add(d1, d2, d);
		dtoc(o, d);
		if(strlen(o) > 80){
			puts("overflow");
			continue;
		}
		printf("%s\n", o);
	}
	return 0;
}

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

int main(){
	int n;
	scanf(" %d", &n);
	
	for(int i=0; i<n; ++i){
		char a[200], b[200], x[200], y[200];
		scanf(" %s %s", a, b);
		
		int LA=strlen(a);
		if(LA>80){
			printf("overflow\n");
			continue;
		}
		for(int i=0; i<LA; ++i){
			x[LA-1-i]=a[i];
		}
		
		int LB=strlen(b);
		if(LB>80){
			printf("overflow\n");
			continue;
		}
		for(int i=0; i<LB; ++i){
			y[LB-1-i]=b[i];
		}
		
		int big, small;
		bool isA;
		if(LA>LB) {big=LA; small=LB; isA=true;}
		else {big=LB; small=LA; isA=false;}
		
		char ans[81];
		int carry=0;
		for(int i=0; i<small; ++i){
			int ta=x[i]-'0', tb=y[i]-'0';
			ta+=tb+carry;
			
			if(ta>=10) carry=1;
			else carry=0;
			ans[i]=(ta%10)+'0';
		}
		if(isA){
			for(int i=small; i<big; ++i){
				int ta=x[i]-'0';
				ta+=carry;
			
				if(ta>=10) carry=1;
				else carry=0;
				ans[i]=(ta%10)+'0';
			}
		}
		else{
			for(int i=small; i<big; ++i){
				int tb=y[i]-'0';
				tb+=carry;
			
				if(tb>=10) carry=1;
				else carry=0;
				ans[i]=(tb%10)+'0';
			}
		}
		
		if(carry!=0){
			ans[big++]=carry+'0';
			ans[big]='\0';
			//printf("carries\n");
		}	
			
		int LS=strlen(ans);
		char fans[200];
		if(LS>80) printf("overflow\n");
		else{
			for(int i=0; i<LS; ++i){
				fans[i]=ans[LS-1-i];
			}		
			printf("%s\n", fans);
		}
	
	}

	return 0;
}