C++ :

#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
const int N = 505;
int a[N],b[N],c[N];

void add(int a[],int b[],int c[]){
	fill(c,c+N,0);
	c[0]=b[0];
	for (int i=1; i<=c[0]; i++){
		c[i]+=a[i]+b[i];
		c[i+1]+=c[i]/10;
		c[i]%=10;
	}
	if (c[c[0]+1]) c[0]++;
}

void fibo(int n){
	if (n<=2) { c[0]=1; c[1]=n; return; }
	a[0]=a[1]=b[0]=1; b[1]=2;
	for (int i=3; i<=n; i++){
		add(a,b,c);
		memcpy(a,b,sizeof(b));
		memcpy(b,c,sizeof(c));
	}
}

void output(int a[]){
	for (int i=a[0]; i>=1; i--) cout<<a[i];
	cout<<endl;
}

int main(){
	int m,n;
	cin>>m>>n;
	fibo(n-m);
	output(c);
	return 0;
}

Pascal :

const maxn=10000;
type sz=array[1..maxn] of longint;
var a,b,c:sz;
    i,j,m,n,lena,lenb,l:longint;
procedure add(var a,b,c:sz);
var i:longint;
begin
  if lena>lenb then l:=lena else l:=lenb;
  for i:=1 to l do c[i]:=a[i]+b[i];
  for i:=1 to l do
    if c[i]>=10then
     begin
        c[i]:=c[i]-10;
        c[i+1]:=c[i+1]+1;
      end;
  if c[l+1]>0 then l:=l+1;
  a:=b;lena:=lenb;b:=c;lenb:=l;
end;
  
begin
  readln(m,n);
  if m>n then exit
    else if n-m=1 then writeln(1)
      else
      begin
          a[1]:=1;b[1]:=1;lena:=1;lenb:=1;l:=0;
          for i:=3 to n-m+1 do
            add(a,b,c);
          for i:=l downto 1 do write(c[i]);
      end;
end.